=head1 NAME

perldiag - various Perl diagnostics

=head1 DESCRIPTION

These messages are classified as follows (listed in increasing order of
desperation):

    (W) A warning (optional).
    (D) A deprecation (enabled by default).
    (S) A severe warning (enabled by default).
    (F) A fatal error (trappable).
    (P) An internal error you should never see (trappable).
    (X) A very fatal error (nontrappable).
    (A) An alien error message (not generated by Perl).

The majority of messages from the first three classifications above
(W, D & S) can be controlled using the C<warnings> pragma.

If a message can be controlled by the C<warnings> pragma, its warning
category is included with the classification letter in the description
below.  E.g. C<(W closed)> means a warning in the C<closed> category.

Optional warnings are enabled by using the C<warnings> pragma or the B<-w>
and B<-W> switches.  Warnings may be captured by setting C<$SIG{__WARN__}>
to a reference to a routine that will be called on each warning instead
of printing it.  See L<perlvar>.

Severe warnings are always enabled, unless they are explicitly disabled
with the C<warnings> pragma or the B<-X> switch.

Trappable errors may be trapped using the eval operator.  See
L<perlfunc/eval>.  In almost all cases, warnings may be selectively
disabled or promoted to fatal errors using the C<warnings> pragma.
See L<warnings>.

The messages are in alphabetical order, without regard to upper or
lower-case.  Some of these messages are generic.  Spots that vary are
denoted with a %s or other printf-style escape.  These escapes are
ignored by the alphabetical order, as are all characters other than
letters.  To look up your message, just ignore anything that is not a
letter.

=over 4

=item accept() on closed socket %s

(W closed) You tried to do an accept on a closed socket.  Did you forget
to check the return value of your socket() call?  See
L<perlfunc/accept>.

=item ADJUST is experimental

(S experimental::class) This warning is emitted if you use the C<ADJUST>
keyword of C<use feature 'class'>.  This keyword is currently
experimental and its behaviour may change in future releases of Perl.

=item Aliasing via reference is experimental

(S experimental::refaliasing) This warning is emitted if you use
a reference constructor on the left-hand side of an assignment to
alias one variable to another.  Simply suppress the warning if you
want to use the feature, but know that in doing so you are taking
the risk of using an experimental feature which may change or be
removed in a future Perl version:

    no warnings "experimental::refaliasing";
    use feature "refaliasing";
    \$x = \$y;

=item '%c' allowed only after types %s in %s

(F) The modifiers '!', '<' and '>' are allowed in pack() or unpack() only
after certain types.  See L<perlfunc/pack>.

=item alpha->numify() is lossy

(W numeric) An alpha version can not be numified without losing
information.

=item Ambiguous call resolved as CORE::%s(), qualify as such or use &

(W ambiguous) A subroutine you have declared has the same name as a Perl
keyword, and you have used the name without qualification for calling
one or the other.  Perl decided to call the builtin because the
subroutine is not imported.

To force interpretation as a subroutine call, either put an ampersand
before the subroutine name, or qualify the name with its package.
Alternatively, you can import the subroutine (or pretend that it's
imported with the C<use subs> pragma).

To silently interpret it as the Perl operator, use the C<CORE::> prefix
on the operator (e.g. C<CORE::log($x)>) or declare the subroutine
to be an object method (see L<perlsub/"Subroutine Attributes"> or
L<attributes>).

=item Ambiguous range in transliteration operator

(F) You wrote something like C<tr/a-z-0//> which doesn't mean anything at
all.  To include a C<-> character in a transliteration, put it either
first or last.  (In the past, C<tr/a-z-0//> was synonymous with
C<tr/a-y//>, which was probably not what you would have expected.)

=item Ambiguous use of %s resolved as %s

(S ambiguous) You said something that may not be interpreted the way
you thought.  Normally it's pretty easy to disambiguate it by supplying
a missing quote, operator, parenthesis pair or declaration.

=item Ambiguous use of -%s resolved as -&%s()

(S ambiguous) You wrote something like C<-foo>, which might be the
string C<"-foo">, or a call to the function C<foo>, negated.  If you meant
the string, just write C<"-foo">.  If you meant the function call,
write C<-foo()>.

=item Ambiguous use of %c resolved as operator %c

(S ambiguous) C<%>, C<&>, and C<*> are both infix operators (modulus,
bitwise and, and multiplication) I<and> initial special characters
(denoting hashes, subroutines and typeglobs), and you said something
like C<*foo * foo> that might be interpreted as either of them.  We
assumed you meant the infix operator, but please try to make it more
clear -- in the example given, you might write C<*foo * foo()> if you
really meant to multiply a glob by the result of calling a function.

=item Ambiguous use of %c{%s} resolved to %c%s

(W ambiguous) You wrote something like C<@{foo}>, which might be
asking for the variable C<@foo>, or it might be calling a function
named foo, and dereferencing it as an array reference.  If you wanted
the variable, you can just write C<@foo>.  If you wanted to call the
function, write C<@{foo()}> ... or you could just not have a variable
and a function with the same name, and save yourself a lot of trouble.

=item Ambiguous use of %c{%s[...]} resolved to %c%s[...]

=item Ambiguous use of %c{%s{...}} resolved to %c%s{...}

(W ambiguous) You wrote something like C<${foo[2]}> (where foo represents
the name of a Perl keyword), which might be looking for element number
2 of the array named C<@foo>, in which case please write C<$foo[2]>, or you
might have meant to pass an anonymous arrayref to the function named
foo, and then do a scalar deref on the value it returns.  If you meant
that, write C<${foo([2])}>.

In regular expressions, the C<${foo[2]}> syntax is sometimes necessary
to disambiguate between array subscripts and character classes.
C</$length[2345]/>, for instance, will be interpreted as C<$length> followed
by the character class C<[2345]>.  If an array subscript is what you
want, you can avoid the warning by changing C</${length[2345]}/> to the
unsightly C</${\$length[2345]}/>, by renaming your array to something
that does not coincide with a built-in keyword, or by simply turning
off warnings with C<no warnings 'ambiguous';>.

=item '|' and '<' may not both be specified on command line

(F) An error peculiar to VMS.  Perl does its own command line
redirection, and found that STDIN was a pipe, and that you also tried to
redirect STDIN using '<'.  Only one STDIN stream to a customer, please.

=item '|' and '>' may not both be specified on command line

(F) An error peculiar to VMS.  Perl does its own command line
redirection, and thinks you tried to redirect stdout both to a file and
into a pipe to another command.  You need to choose one or the other,
though nothing's stopping you from piping into a program or Perl script
which 'splits' output into two streams, such as

    open(OUT,">$ARGV[0]") or die "Can't write to $ARGV[0]: $!";
    while (<STDIN>) {
        print;
        print OUT;
    }
    close OUT;

=item Applying %s to %s will act on scalar(%s)

(W misc) The pattern match (C<//>), substitution (C<s///>), and
transliteration (C<tr///>) operators work on scalar values.  If you apply
one of them to an array or a hash, it will convert the array or hash to
a scalar value (the length of an array, or the population info of a
hash) and then work on that scalar value.  This is probably not what
you meant to do.  See L<perlfunc/grep> and L<perlfunc/map> for
alternatives.

=item Arg too short for msgsnd

(F) msgsnd() requires a string at least as long as sizeof(long).

=item Argument "%s" isn't numeric%s

(W numeric) The indicated string was fed as an argument to an operator
that expected a numeric value instead.  If you're fortunate the message
will identify which operator was so unfortunate.

Note that for the C<Inf> and C<NaN> (infinity and not-a-number) the
definition of "numeric" is somewhat unusual: the strings themselves
(like "Inf") are considered numeric, and anything following them is
considered non-numeric.

=item Argument list not closed for PerlIO layer "%s"

(W layer) When pushing a layer with arguments onto the Perl I/O
system you forgot the ) that closes the argument list.  (Layers
take care of transforming data between external and internal
representations.)  Perl stopped parsing the layer list at this
point and did not attempt to push this layer.  If your program
didn't explicitly request the failing operation, it may be the
result of the value of the environment variable PERLIO.

=item Argument "%s" treated as 0 in increment (++)

(W numeric) The indicated string was fed as an argument to the C<++>
operator which expects either a number or a string matching
C</^[a-zA-Z]*[0-9]*\z/>.  See L<perlop/Auto-increment and
Auto-decrement> for details.

=item Array passed to stat will be coerced to a scalar%s

(W syntax) You called stat() on an array, but the array will be
coerced to a scalar - the number of elements in the array.

=item A signature parameter must start with '$', '@' or '%'

(F) Each subroutine signature parameter declaration must start with a valid
sigil; for example:

    sub foo ($a, $, $b = 1, @c) {}

=item A slurpy parameter may not have a default value

(F) Only scalar subroutine signature parameters may have a default value;
for example:

    sub foo ($a = 1)        {} # legal
    sub foo (@a = (1))      {} # invalid
    sub foo (%a = (a => b)) {} # invalid

=item assertion botched: %s

(X) The malloc package that comes with Perl had an internal failure.

=item Assertion %s failed: file "%s", line %d

(X) A general assertion failed.  The file in question must be examined.

=item Assigned value is not a reference

(F) You tried to assign something that was not a reference to an lvalue
reference (e.g., C<\$x = $y>).  If you meant to make $x an alias to $y, use
C<\$x = \$y>.

=item Assigned value is not %s reference

(F) You tried to assign a reference to a reference constructor, but the
two references were not of the same type.  You cannot alias a scalar to
an array, or an array to a hash; the two types must match.

    \$x = \@y;  # error
    \@x = \%y;  # error
     $y = [];
    \$x = $y;   # error; did you mean \$y?

=item Assigning non-zero to $[ is no longer possible

(F) When the "array_base" feature is disabled
(e.g., and under C<use v5.16;>, and as of Perl 5.30)
the special variable C<$[>, which is deprecated, is now a fixed zero value.

=item Assignment to both a list and a scalar

(F) If you assign to a conditional operator, the 2nd and 3rd arguments
must either both be scalars or both be lists.  Otherwise Perl won't
know which context to supply to the right side.

=item Assuming NOT a POSIX class since %s in regex; marked by S<<-- HERE> in m/%s/

(W regexp) You had something like these:

 [[:alnum]]
 [[:digit:xyz]

They look like they might have been meant to be the POSIX classes
C<[:alnum:]> or C<[:digit:]>.  If so, they should be written:

 [[:alnum:]]
 [[:digit:]xyz]

Since these aren't legal POSIX class specifications, but are legal
bracketed character classes, Perl treats them as the latter.  In the
first example, it matches the characters C<":">, C<"[">, C<"a">, C<"l">,
C<"m">, C<"n">, and C<"u">.

If these weren't meant to be POSIX classes, this warning message is
spurious, and can be suppressed by reordering things, such as

 [[al:num]]

or

 [[:munla]]

=item <> at require-statement should be quotes

(F) You wrote C<< require <file> >> when you should have written
C<require 'file'>.

=item Attempt to access disallowed key '%s' in a restricted hash

(F) The failing code has attempted to get or set a key which is not in
the current set of allowed keys of a restricted hash.

=item Attempt to bless into a freed package

(F) You wrote C<bless $foo> with one argument after somehow causing
the current package to be freed.  Perl cannot figure out what to
do, so it throws up its hands in despair.

=item Attempt to bless into a class

(F) You are attempting to call C<bless> with a package name that is a
new-style C<class>.  This is not necessary, as instances created by the
constructor are already in the correct class.  Instances cannot be created
by other means, such as C<bless>.

=item Attempt to bless into a reference

(F) The CLASSNAME argument to the bless() operator is expected to be
the name of the package to bless the resulting object into.  You've
supplied instead a reference to something: perhaps you wrote

    bless $self, $proto;

when you intended

    bless $self, ref($proto) || $proto;

If you actually want to bless into the stringified version
of the reference supplied, you need to stringify it yourself, for
example by:

    bless $self, "$proto";

=item Attempt to clear deleted array

(S debugging) An array was assigned to when it was being freed.
Freed values are not supposed to be visible to Perl code.  This
can also happen if XS code calls C<av_clear> from a custom magic
callback on the array.

=item Attempt to delete disallowed key '%s' from a restricted hash

(F) The failing code attempted to delete from a restricted hash a key
which is not in its key set.

=item Attempt to delete readonly key '%s' from a restricted hash

(F) The failing code attempted to delete a key whose value has been
declared readonly from a restricted hash.

=item Attempt to free non-arena SV: 0x%x

(S internal) All SV objects are supposed to be allocated from arenas
that will be garbage collected on exit.  An SV was discovered to be
outside any of those arenas.

=item Attempt to free nonexistent shared string '%s'%s

(S internal) Perl maintains a reference-counted internal table of
strings to optimize the storage and access of hash keys and other
strings.  This indicates someone tried to decrement the reference count
of a string that can no longer be found in the table.

=item Attempt to free temp prematurely: SV 0x%x

(S debugging) Mortalized values are supposed to be freed by the
free_tmps() routine.  This indicates that something else is freeing the
SV before the free_tmps() routine gets a chance, which means that the
free_tmps() routine will be freeing an unreferenced scalar when it does
try to free it.

=item Attempt to free unreferenced glob pointers

(S internal) The reference counts got screwed up on symbol aliases.

=item Attempt to free unreferenced scalar: SV 0x%x

(S internal) Perl went to decrement the reference count of a scalar to
see if it would go to 0, and discovered that it had already gone to 0
earlier, and should have been freed, and in fact, probably was freed.
This could indicate that SvREFCNT_dec() was called too many times, or
that SvREFCNT_inc() was called too few times, or that the SV was
mortalized when it shouldn't have been, or that memory has been
corrupted.

=item Attempt to pack pointer to temporary value

(W pack) You tried to pass a temporary value (like the result of a
function, or a computed expression) to the "p" pack() template.  This
means the result contains a pointer to a location that could become
invalid anytime, even before the end of the current statement.  Use
literals or global values as arguments to the "p" pack() template to
avoid this warning.

=item Attempt to reload %s aborted.

(F) You tried to load a file with C<use> or C<require> that failed to
compile once already.  Perl will not try to compile this file again
unless you delete its entry from %INC.  See L<perlfunc/require> and
L<perlvar/%INC>.

=item Attempt to set length of freed array

(W misc) You tried to set the length of an array which has
been freed.  You can do this by storing a reference to the
scalar representing the last index of an array and later
assigning through that reference.  For example

    $r = do {my @a; \$#a};
    $$r = 503

=item Attempt to use reference as lvalue in substr

(W substr) You supplied a reference as the first argument to substr()
used as an lvalue, which is pretty strange.  Perhaps you forgot to
dereference it first.  See L<perlfunc/substr>.

=item Attribute prototype(%s) discards earlier prototype attribute in same sub

(W misc) A sub was declared as sub foo : prototype(A) : prototype(B) {}, for
example.  Since each sub can only have one prototype, the earlier
declaration(s) are discarded while the last one is applied.

=item av_reify called on tied array

(S debugging) This indicates that something went wrong and Perl got I<very>
confused about C<@_> or C<@DB::args> being tied.

=item Bad arg length for %s, is %u, should be %d

(F) You passed a buffer of the wrong size to one of msgctl(), semctl()
or shmctl().  In C parlance, the correct sizes are, respectively,
S<sizeof(struct msqid_ds *)>, S<sizeof(struct semid_ds *)>, and
S<sizeof(struct shmid_ds *)>.

=item Bad evalled substitution pattern

(F) You've used the C</e> switch to evaluate the replacement for a
substitution, but perl found a syntax error in the code to evaluate,
most likely an unexpected right brace '}'.

=item Bad filehandle: %s

(F) A symbol was passed to something wanting a filehandle, but the
symbol has no filehandle associated with it.  Perhaps you didn't do an
open(), or did it in another package.

=item Bad free() ignored

(S malloc) An internal routine called free() on something that had never
been malloc()ed in the first place.  Mandatory, but can be disabled by
setting environment variable C<PERL_BADFREE> to 0.

This message can be seen quite often with DB_File on systems with "hard"
dynamic linking, like C<AIX> and C<OS/2>.  It is a bug of C<Berkeley DB>
which is left unnoticed if C<DB> uses I<forgiving> system malloc().

=item Bad infix plugin result (%zd) - did not consume entire identifier <%s>

(F) A plugin using the C<PL_infix_plugin> mechanism to parse an infix
keyword consumed part of a named identifier operator name but did not
consume all of it.  This is not permitted as it leads to fragile parsing
results.

=item Badly placed ()'s

(A) You've accidentally run your script through B<csh> instead
of Perl.  Check the #! line, or manually feed your script into
Perl yourself.

=item Bad name after %s

(F) You started to name a symbol by using a package prefix, and then
didn't finish the symbol.  In particular, you can't interpolate outside
of quotes, so

    $var = 'myvar';
    $sym = mypack::$var;

is not the same as

    $var = 'myvar';
    $sym = "mypack::$var";

=item Bad plugin affecting keyword '%s'

(F) An extension using the keyword plugin mechanism violated the
plugin API.

=item Bad realloc() ignored

(S malloc) An internal routine called realloc() on something that
had never been malloc()ed in the first place.  Mandatory, but can
be disabled by setting the environment variable C<PERL_BADFREE> to 1.

=item Bad symbol for %s

(P) An internal request asked to add an entry of the named type to something that
wasn't a symbol table entry.

=item Bad symbol for scalar

(P) An internal request asked to add a scalar entry to something that
wasn't a symbol table entry.

=item Bareword found in conditional

(W bareword) The compiler found a bareword where it expected a
conditional, which often indicates that an || or && was parsed as part
of the last argument of the previous construct, for example:

    open FOO || die;

It may also indicate a misspelled constant that has been interpreted as
a bareword:

    use constant TYPO => 1;
    if (TYOP) { print "foo" }

The C<strict> pragma is useful in avoiding such errors.

=item Bareword in require contains "%s"

=item Bareword in require maps to disallowed filename "%s"

=item Bareword in require maps to empty filename

(F) The bareword form of require has been invoked with a filename which could
not have been generated by a valid bareword permitted by the parser.  You
shouldn't be able to get this error from Perl code, but XS code may throw it
if it passes an invalid module name to C<Perl_load_module>.

=item Bareword in require must not start with a double-colon: "%s"

(F) In C<require Bare::Word>, the bareword is not allowed to start with a
double-colon.  Write C<require ::Foo::Bar> as  C<require Foo::Bar> instead.

=item Bareword "%s" not allowed while "strict subs" in use

(F) With "strict subs" in use, a bareword is only allowed as a
subroutine identifier, in curly brackets or to the left of the "=>"
symbol.  Perhaps you need to predeclare a subroutine?

=item Bareword "%s" refers to nonexistent package

(W bareword) You used a qualified bareword of the form C<Foo::>, but the
compiler saw no other uses of that namespace before that point.  Perhaps
you need to predeclare a package?

=item Bareword filehandle "%s" not allowed under 'no feature "bareword_filehandles"'

(F) You attempted to use a bareword filehandle with the
C<bareword_filehandles> feature disabled.

Only the built-in handles C<STDIN>, C<STDOUT>, C<STDERR>, C<ARGV>,
C<ARGVOUT> and C<DATA> can be used with the C<bareword_filehandles>
feature disabled.

=item BEGIN failed--compilation aborted

(F) An untrapped exception was raised while executing a BEGIN
subroutine.  Compilation stops immediately and the interpreter is
exited.

=item BEGIN not safe after errors--compilation aborted

(F) Perl found a C<BEGIN {}> subroutine (or a C<use> directive, which
implies a C<BEGIN {}>) after one or more compilation errors had already
occurred.  Since the intended environment for the C<BEGIN {}> could not
be guaranteed (due to the errors), and since subsequent code likely
depends on its correct operation, Perl just gave up.

=item \%d better written as $%d

(W syntax) Outside of patterns, backreferences live on as variables.
The use of backslashes is grandfathered on the right-hand side of a
substitution, but stylistically it's better to use the variable form
because other Perl programmers will expect it, and it works better if
there are more than 9 backreferences.

=item Binary number > 0b11111111111111111111111111111111 non-portable

(W portable) The binary number you specified is larger than 2**32-1
(4294967295) and therefore non-portable between systems.  See
L<perlport> for more on portability concerns.

=item bind() on closed socket %s

(W closed) You tried to do a bind on a closed socket.  Did you forget to
check the return value of your socket() call?  See L<perlfunc/bind>.

=item binmode() on closed filehandle %s

(W unopened) You tried binmode() on a filehandle that was never opened.
Check your control flow and number of arguments.

=item Bit vector size > 32 non-portable

(W portable) Using bit vector sizes larger than 32 is non-portable.

=item Bizarre copy of %s

(P) Perl detected an attempt to copy an internal value that is not
copiable.

=item Bizarre SvTYPE [%d]

(P) When starting a new thread or returning values from a thread, Perl
encountered an invalid data type.

=item Both or neither range ends should be Unicode in regex; marked by
S<<-- HERE> in m/%s/

(W regexp) (only under C<S<use re 'strict'>> or within C<(?[...])>)

In a bracketed character class in a regular expression pattern, you
had a range which has exactly one end of it specified using C<\N{}>, and
the other end is specified using a non-portable mechanism.  Perl treats
the range as a Unicode range, that is, all the characters in it are
considered to be the Unicode characters, and which may be different code
points on some platforms Perl runs on.  For example, C<[\N{U+06}-\x08]>
is treated as if you had instead said C<[\N{U+06}-\N{U+08}]>, that is it
matches the characters whose code points in Unicode are 6, 7, and 8.
But that C<\x08> might indicate that you meant something different, so
the warning gets raised.

=item Buffer overflow in prime_env_iter: %s

(W internal) A warning peculiar to VMS.  While Perl was preparing to
iterate over %ENV, it encountered a logical name or symbol definition
which was too long, so it was truncated to the string shown.

=item Built-in function '%s' is experimental

(S experimental::builtin) A call is being made to a function in the
C<builtin::> namespace, which is currently experimental. The existence
or nature of the function may be subject to change in a future version
of Perl.

=item builtin::import can only be called at compile time

(F) The C<import> method of the C<builtin> package was invoked when no code
is currently being compiled. Since this method is used to introduce new
lexical subroutines into the scope currently being compiled, this is not
going to have any effect.

=item Callback called exit

(F) A subroutine invoked from an external package via call_sv()
exited by calling exit.

=item %s() called too early to check prototype

(W prototype) You've called a function that has a prototype before the
parser saw a definition or declaration for it, and Perl could not check
that the call conforms to the prototype.  You need to either add an
early prototype declaration for the subroutine in question, or move the
subroutine definition ahead of the call to get proper prototype
checking.  Alternatively, if you are certain that you're calling the
function correctly, you may put an ampersand before the name to avoid
the warning.  See L<perlsub>.

=item Cannot assign :param(%s) to field %s because that name is already in use

(F) An attempt was made to apply a parameter name to a field, when the name
is already being used by another field in the same class, or one of its
parent classes. This would cause a name clash so is not allowed.

=item Cannot chr %f

(F) You passed an invalid number (like an infinity or not-a-number) to C<chr>.

=item Cannot complete in-place edit of %s: %s

(F) Your perl script appears to have changed directory while
performing an in-place edit of a file specified by a relative path,
and your system doesn't include the directory relative POSIX functions
needed to handle that.

=item Cannot compress %f in pack

(F) You tried compressing an infinity or not-a-number as an unsigned
integer with BER, which makes no sense.

=item Cannot compress integer in pack

(F) An argument to pack("w",...) was too large to compress.
The BER compressed integer format can only be used with positive
integers, and you attempted to compress a very large number (> 1e308).
See L<perlfunc/pack>.

=item Cannot compress negative numbers in pack

(F) An argument to pack("w",...) was negative.  The BER compressed integer
format can only be used with positive integers.  See L<perlfunc/pack>.

=item Cannot convert a reference to %s to typeglob

(F) You manipulated Perl's symbol table directly, stored a reference
in it, then tried to access that symbol via conventional Perl syntax.
The access triggers Perl to autovivify that typeglob, but it there is
no legal conversion from that type of reference to a typeglob.

=item Cannot copy to %s

(P) Perl detected an attempt to copy a value to an internal type that cannot
be directly assigned to.

=item Cannot create class %s as it already has a non-empty @ISA

(F) An attempt was made to create a class out of a package that already has
an C<@ISA> array, and the array is not empty.  This is not permitted, as it
would lead to a class with inconsistent inheritance.

=item Cannot find encoding "%s"

(S io) You tried to apply an encoding that did not exist to a filehandle,
either with open() or binmode().

=item Cannot invoke a method of "%s" on an instance of "%s"

(F) You tried to directly call a C<method> subroutine of one class by passing
in a value that is an instance of a different class.  This is not permitted,
as the method would not have access to the correct instance fields.

=item Cannot invoke method on a non-instance

(F) You tried to directly call a C<method> subroutine of a class by passing
in a value that is not an instance of that class.  This is not permitted, as
the method would not then have access to its instance fields.

=item Cannot open %s as a dirhandle: it is already open as a filehandle

(F) You tried to use opendir() to associate a dirhandle to a symbol (glob
or scalar) that already holds a filehandle.  Since this idiom might render
your code confusing, it was deprecated in Perl 5.10.  As of Perl 5.28, it
is a fatal error.

=item Cannot open %s as a filehandle: it is already open as a dirhandle

(F) You tried to use open() to associate a filehandle to a symbol (glob
or scalar) that already holds a dirhandle.  Since this idiom might render
your code confusing, it was deprecated in Perl 5.10.  As of Perl 5.28, it
is a fatal error.

=item Cannot '%s' outside of a 'class'

(F) You attempted to use one of the keywords that only makes sense inside
a C<class> definition, at a location that is not inside such a class.

=item Cannot pack %f with '%c'

(F) You tried converting an infinity or not-a-number to an integer,
which makes no sense.

=item Cannot printf %f with '%c'

(F) You tried printing an infinity or not-a-number as a character (%c),
which makes no sense.  Maybe you meant '%s', or just stringifying it?

=item Cannot reopen existing class "%s"

(F) You tried to begin a C<class> definition for a class that already exists.
A class may only have one definition block.

=item Cannot set tied @DB::args

(F) C<caller> tried to set C<@DB::args>, but found it tied.  Tying C<@DB::args>
is not supported.  (Before this error was added, it used to crash.)

=item Cannot tie unreifiable array

(P) You somehow managed to call C<tie> on an array that does not
keep a reference count on its arguments and cannot be made to
do so.  Such arrays are not even supposed to be accessible to
Perl code, but are only used internally.

=item Cannot yet reorder sv_vcatpvfn() arguments from va_list

(F) Some XS code tried to use C<sv_vcatpvfn()> or a related function with a
format string that specifies explicit indexes for some of the elements, and
using a C-style variable-argument list (a C<va_list>).  This is not currently
supported.  XS authors wanting to do this must instead construct a C array
of C<SV*> scalars containing the arguments.

=item Can only compress unsigned integers in pack

(F) An argument to pack("w",...) was not an integer.  The BER compressed
integer format can only be used with positive integers, and you attempted
to compress something else.  See L<perlfunc/pack>.

=item Can't "%s" out of a "defer" block

(F) An attempt was made to jump out of the scope of a C<defer> block by using
a control-flow statement such as C<return>, C<goto> or a loop control. This is
not permitted.

=item Can't "%s" out of a "finally" block

(F) Similar to above, but involving a C<finally> block at the end of a
C<try>/C<catch> construction rather than a C<defer> block.

=item Can't bless an object reference

(F) You attempted to call C<bless> on a value that already refers to a real
object instance.

=item Can't bless non-reference value

(F) Only hard references may be blessed.  This is how Perl "enforces"
encapsulation of objects.  See L<perlobj>.

=item Can't "break" in a loop topicalizer

(F) You called C<break>, but you're in a C<foreach> block rather than
a C<given> block.  You probably meant to use C<next> or C<last>.

=item Can't "break" outside a given block

(F) You called C<break>, but you're not inside a C<given> block.

=item Can't call destructor for 0x%p in global destruction

(S) This should not happen. Internals code has set up a destructor
using C<mortal_destructor_sv> or C<mortal_destructor_x> which is firing
during global destruction. Please attempt to reduce the code that triggers
this warning down to a small an example as possible and then report the
problem to L<https://github.com/Perl/perl5/issues/new/choose>

=item Can't call method "%s" on an undefined value

(F) You used the syntax of a method call, but the slot filled by the
object reference or package name contains an undefined value.  Something
like this will reproduce the error:

    $BADREF = undef;
    process $BADREF 1,2,3;
    $BADREF->process(1,2,3);

=item Can't call method "%s" on unblessed reference

(F) A method call must know in what package it's supposed to run.  It
ordinarily finds this out from the object reference you supply, but you
didn't supply an object reference in this case.  A reference isn't an
object reference until it has been blessed.  See L<perlobj>.

=item Can't call method "%s" without a package or object reference

(F) You used the syntax of a method call, but the slot filled by the
object reference or package name contains an expression that returns a
defined value which is neither an object reference nor a package name.
Something like this will reproduce the error:

    $BADREF = 42;
    process $BADREF 1,2,3;
    $BADREF->process(1,2,3);

=item Can't call mro_isa_changed_in() on anonymous symbol table

(P) Perl got confused as to whether a hash was a plain hash or a
symbol table hash when trying to update @ISA caches.

=item Can't call mro_method_changed_in() on anonymous symbol table

(F) An XS module tried to call C<mro_method_changed_in> on a hash that was
not attached to the symbol table.

=item Can't chdir to %s

(F) You called C<perl -x/foo/bar>, but F</foo/bar> is not a directory
that you can chdir to, possibly because it doesn't exist.

=item Can't coerce %s to %s in %s

(F) Certain types of SVs, in particular real symbol table entries
(typeglobs), can't be forced to stop being what they are.  So you can't
say things like:

    *foo += 1;

You CAN say

    $foo = *foo;
    $foo += 1;

but then $foo no longer contains a glob.

=item Can't "continue" outside a when block

(F) You called C<continue>, but you're not inside a C<when>
or C<default> block.

=item can't convert empty path

(F) On Cygwin, you called a path conversion function with an empty path.
Only non-empty paths are legal.

=item Can't create pipe mailbox

(P) An error peculiar to VMS.  The process is suffering from exhausted
quotas or other plumbing problems.

=item Can't declare %s in "%s"

(F) Only scalar, array, and hash variables may be declared as "my", "our" or
"state" variables.  They must have ordinary identifiers as names.

=item Can't "default" outside a topicalizer

(F) You have used a C<default> block that is neither inside a
C<foreach> loop nor a C<given> block.  (Note that this error is
issued on exit from the C<default> block, so you won't get the
error if you use an explicit C<continue>.)

=item Can't determine class of operator %s, assuming BASEOP

(S) This warning indicates something wrong in the internals of perl.
Perl was trying to find the class (e.g. LISTOP) of a particular OP,
and was unable to do so. This is likely to be due to a bug in the perl
internals, or due to a bug in XS code which manipulates perl optrees.

=item Can't do inplace edit: %s is not a regular file

(S inplace) You tried to use the B<-i> switch on a special file, such as
a file in /dev, a FIFO or an uneditable directory.  The file was ignored.

=item Can't do inplace edit on %s: %s

(S inplace) The creation of the new file failed for the indicated
reason.

=item Can't do inplace edit: %s would not be unique

(S inplace) Your filesystem does not support filenames longer than 14
characters and Perl was unable to create a unique filename during
inplace editing with the B<-i> switch.  The file was ignored.

=item Can't do %s("%s") on non-UTF-8 locale; resolved to "%s".

(W locale) You are 1) running under "C<use locale>"; 2) the current
locale is not a UTF-8 one; 3) you tried to do the designated case-change
operation on the specified Unicode character; and 4) the result of this
operation would mix Unicode and locale rules, which likely conflict.
Mixing of different rule types is forbidden, so the operation was not
done; instead the result is the indicated value, which is the best
available that uses entirely Unicode rules.  That turns out to almost
always be the original character, unchanged.

It is generally a bad idea to mix non-UTF-8 locales and Unicode, and
this issue is one of the reasons why.  This warning is raised when
Unicode rules would normally cause the result of this operation to
contain a character that is in the range specified by the locale,
0..255, and hence is subject to the locale's rules, not Unicode's.

If you are using locale purely for its characteristics related to things
like its numeric and time formatting (and not C<LC_CTYPE>), consider
using a restricted form of the locale pragma (see L<perllocale/The "use
locale" pragma>) like "S<C<use locale ':not_characters'>>".

Note that failed case-changing operations done as a result of
case-insensitive C</i> regular expression matching will show up in this
warning as having the C<fc> operation (as that is what the regular
expression engine calls behind the scenes.)

=item Can't do waitpid with flags

(F) This machine doesn't have either waitpid() or wait4(), so only
waitpid() without flags is emulated.

=item Can't emulate -%s on #! line

(F) The #! line specifies a switch that doesn't make sense at this
point.  For example, it'd be kind of silly to put a B<-x> on the #!
line.

=item Can't %s %s-endian %ss on this platform

(F) Your platform's byte-order is neither big-endian nor little-endian,
or it has a very strange pointer size.  Packing and unpacking big- or
little-endian floating point values and pointers may not be possible.
See L<perlfunc/pack>.

=item Can't exec "%s": %s

(W exec) A system(), exec(), or piped open call could not execute the
named program for the indicated reason.  Typical reasons include: the
permissions were wrong on the file, the file wasn't found in
C<$ENV{PATH}>, the executable in question was compiled for another
architecture, or the #! line in a script points to an interpreter that
can't be run for similar reasons.  (Or maybe your system doesn't support
#! at all.)

=item Can't exec %s

(F) Perl was trying to execute the indicated program for you because
that's what the #! line said.  If that's not what you wanted, you may
need to mention "perl" on the #! line somewhere.

=item Can't execute %s

(F) You used the B<-S> switch, but the copies of the script to execute
found in the PATH did not have correct permissions.

=item Can't find an opnumber for "%s"

(F) A string of a form C<CORE::word> was given to prototype(), but there
is no builtin with the name C<word>.

=item Can't find label %s

(F) You said to goto a label that isn't mentioned anywhere that it's
possible for us to go to.  See L<perlfunc/goto>.

=item Can't find %s on PATH

(F) You used the B<-S> switch, but the script to execute could not be
found in the PATH.

=item Can't find %s on PATH, '.' not in PATH

(F) You used the B<-S> switch, but the script to execute could not be
found in the PATH, or at least not with the correct permissions.  The
script exists in the current directory, but PATH prohibits running it.

=item Can't find string terminator %s anywhere before EOF

(F) Perl strings can stretch over multiple lines.  This message means
that the closing delimiter was omitted.  Because bracketed quotes count
nesting levels, the following is missing its final parenthesis:

    print q(The character '(' starts a side comment.);

If you're getting this error from a here-document, you may have
included unseen whitespace before or after your closing tag or there
may not be a linebreak after it.  A good programmer's editor will have
a way to help you find these characters (or lack of characters).  See
L<perlop> for the full details on here-documents.

=item Can't find Unicode property definition "%s"

=item Can't find Unicode property definition "%s" in regex; marked by <-- HERE in m/%s/

(F) The named property which you specified via C<\p> or C<\P> is not one
known to Perl.  Perhaps you misspelled the name?  See
L<perluniprops/Properties accessible through \p{} and \P{}>
for a complete list of available official
properties.  If it is a
L<user-defined property|perlunicode/User-Defined Character Properties>
it must have been defined by the time the regular expression is
matched.

If you didn't mean to use a Unicode property, escape the C<\p>, either
by C<\\p> (just the C<\p>) or by C<\Q\p> (the rest of the string, or
until C<\E>).

=item Can't fork: %s

(F) A fatal error occurred while trying to fork while opening a
pipeline.

=item Can't fork, trying again in 5 seconds

(W pipe) A fork in a piped open failed with EAGAIN and will be retried
after five seconds.

=item Can't get filespec - stale stat buffer?

(S) A warning peculiar to VMS.  This arises because of the difference
between access checks under VMS and under the Unix model Perl assumes.
Under VMS, access checks are done by filename, rather than by bits in
the stat buffer, so that ACLs and other protections can be taken into
account.  Unfortunately, Perl assumes that the stat buffer contains all
the necessary information, and passes it, instead of the filespec, to
the access-checking routine.  It will try to retrieve the filespec using
the device name and FID present in the stat buffer, but this works only
if you haven't made a subsequent call to the CRTL stat() routine,
because the device name is overwritten with each call.  If this warning
appears, the name lookup failed, and the access-checking routine gave up
and returned FALSE, just to be conservative.  (Note: The access-checking
routine knows about the Perl C<stat> operator and file tests, so you
shouldn't ever see this warning in response to a Perl command; it arises
only if some internal code takes stat buffers lightly.)

=item Can't get pipe mailbox device name

(P) An error peculiar to VMS.  After creating a mailbox to act as a
pipe, Perl can't retrieve its name for later use.

=item Can't get SYSGEN parameter value for MAXBUF

(P) An error peculiar to VMS.  Perl asked $GETSYI how big you want your
mailbox buffers to be, and didn't get an answer.

=item Can't "goto" into a binary or list expression

(F) A "goto" statement was executed to jump into the middle of a binary
or list expression.  You can't get there from here.  The reason for this
restriction is that the interpreter would get confused as to how many
arguments there are, resulting in stack corruption or crashes.  This
error occurs in cases such as these:

    goto F;
    print do { F: }; # Can't jump into the arguments to print

    goto G;
    $x + do { G: $y }; # How is + supposed to get its first operand?

=item Can't "goto" into a "defer" block

(F) A C<goto> statement was executed to jump into the scope of a C<defer>
block.  This is not permitted.

=item Can't "goto" into a "given" block

(F) A "goto" statement was executed to jump into the middle of a C<given>
block.  You can't get there from here.  See L<perlfunc/goto>.

=item Can't "goto" into the middle of a foreach loop

(F) A "goto" statement was executed to jump into the middle of a foreach
loop.  You can't get there from here.  See L<perlfunc/goto>.

=item Can't "goto" out of a pseudo block

(F) A "goto" statement was executed to jump out of what might look like
a block, except that it isn't a proper block.  This usually occurs if
you tried to jump out of a sort() block or subroutine, which is a no-no.
See L<perlfunc/goto>.

=item Can't goto subroutine from an eval-%s

(F) The "goto subroutine" call can't be used to jump out of an eval
"string" or block.

=item Can't goto subroutine from a sort sub (or similar callback)

(F) The "goto subroutine" call can't be used to jump out of the
comparison sub for a sort(), or from a similar callback (such
as the reduce() function in List::Util).

=item Can't goto subroutine outside a subroutine

(F) The deeply magical "goto subroutine" call can only replace one
subroutine call for another.  It can't manufacture one out of whole
cloth.  In general you should be calling it out of only an AUTOLOAD
routine anyway.  See L<perlfunc/goto>.

=item Can't ignore signal CHLD, forcing to default

(W signal) Perl has detected that it is being run with the SIGCHLD
signal (sometimes known as SIGCLD) disabled.  Since disabling this
signal will interfere with proper determination of exit status of child
processes, Perl has reset the signal to its default value.  This
situation typically indicates that the parent program under which Perl
may be running (e.g. cron) is being very careless.

=item Can't kill a non-numeric process ID

(F) Process identifiers must be (signed) integers.  It is a fatal error to
attempt to kill() an undefined, empty-string or otherwise non-numeric
process identifier.

=item Can't "last" outside a loop block

(F) A "last" statement was executed to break out of the current block,
except that there's this itty bitty problem called there isn't a current
block.  Note that an "if" or "else" block doesn't count as a "loopish"
block, as doesn't a block given to sort(), map() or grep().  You can
usually double the curlies to get the same effect though, because the
inner curlies will be considered a block that loops once.  See
L<perlfunc/last>.

=item Can't linearize anonymous symbol table

(F) Perl tried to calculate the method resolution order (MRO) of a
package, but failed because the package stash has no name.

=item Can't load '%s' for module %s

(F) The module you tried to load failed to load a dynamic extension.
This may either mean that you upgraded your version of perl to one
that is incompatible with your old dynamic extensions (which is known
to happen between major versions of perl), or (more likely) that your
dynamic extension was built against an older version of the library
that is installed on your system.  You may need to rebuild your old
dynamic extensions.

=item Can't localize lexical variable %s

(F) You used local on a variable name that was previously declared as a
lexical variable using "my" or "state".  This is not allowed.  If you
want to localize a package variable of the same name, qualify it with
the package name.

=item Can't localize through a reference

(F) You said something like C<local $$ref>, which Perl can't currently
handle, because when it goes to restore the old value of whatever $ref
pointed to after the scope of the local() is finished, it can't be sure
that $ref will still be a reference.

=item Can't locate %s

(F) You said to C<do> (or C<require>, or C<use>) a file that couldn't be found.
Perl looks for the file in all the locations mentioned in @INC, unless
the file name included the full path to the file.  Perhaps you need
to set the PERL5LIB or PERL5OPT environment variable to say where the
extra library is, or maybe the script needs to add the library name
to @INC.  Or maybe you just misspelled the name of the file.  See
L<perlfunc/require> and L<lib>.

=item Can't locate auto/%s.al in @INC

(F) A function (or method) was called in a package which allows
autoload, but there is no function to autoload.  Most probable causes
are a misprint in a function/method name or a failure to C<AutoSplit>
the file, say, by doing C<make install>.

=item Can't locate loadable object for module %s in @INC

(F) The module you loaded is trying to load an external library, like
for example, F<foo.so> or F<bar.dll>, but the L<DynaLoader> module was
unable to locate this library.  See L<DynaLoader>.

=item Can't locate object method "%s" via package "%s"

(F) You called a method correctly, and it correctly indicated a package
functioning as a class, but that package doesn't define that particular
method, nor does any of its base classes.  See L<perlobj>.

=item Can't locate object method "%s" via package "%s" (perhaps you forgot
to load "%s"?)

(F) You called a method on a class that did not exist, and the method
could not be found in UNIVERSAL.  This often means that a method
requires a package that has not been loaded.

=item Can't locate object method "INC", nor "INCDIR" nor string overload via package "%s" %s in @INC

(F) You pushed an object, either directly or via an array reference hook,
into C<@INC>, but the object doesn't support any known hook methods, nor
a string overload and is also not a blessed CODE reference. In short the
C<require> function does not know what to do with the object.
See also L<perlfunc/require>.

=item Can't locate package %s for @%s::ISA

(W syntax) The @ISA array contained the name of another package that
doesn't seem to exist.

=item Can't locate PerlIO%s

(F) You tried to use in open() a PerlIO layer that does not exist,
e.g. open(FH, ">:nosuchlayer", "somefile").

=item Can't make list assignment to %ENV on this system

(F) List assignment to %ENV is not supported on some systems, notably
VMS.

=item Can't make loaded symbols global on this platform while loading %s

(S) A module passed the flag 0x01 to DynaLoader::dl_load_file() to request
that symbols from the stated file are made available globally within the
process, but that functionality is not available on this platform.  Whilst
the module likely will still work, this may prevent the perl interpreter
from loading other XS-based extensions which need to link directly to
functions defined in the C or XS code in the stated file.

=item Can't modify %s in %s

(F) You aren't allowed to assign to the item indicated, or otherwise try
to change it, such as with an auto-increment.

=item Can't modify non-lvalue subroutine call of &%s

=item Can't modify non-lvalue subroutine call of &%s in %s

(F) Subroutines meant to be used in lvalue context should be declared as
such.  See L<perlsub/"Lvalue subroutines">.

=item Can't modify reference to %s in %s assignment

(F) Only a limited number of constructs can be used as the argument to a
reference constructor on the left-hand side of an assignment, and what
you used was not one of them.  See L<perlref/Assigning to References>.

=item Can't modify reference to localized parenthesized array in list
assignment

(F) Assigning to C<\local(@array)> or C<\(local @array)> is not supported, as
it is not clear exactly what it should do.  If you meant to make @array
refer to some other array, use C<\@array = \@other_array>.  If you want to
make the elements of @array aliases of the scalars referenced on the
right-hand side, use C<\(@array) = @scalar_refs>.

=item Can't modify reference to parenthesized hash in list assignment

(F) Assigning to C<\(%hash)> is not supported.  If you meant to make %hash
refer to some other hash, use C<\%hash = \%other_hash>.  If you want to
make the elements of %hash into aliases of the scalars referenced on the
right-hand side, use a hash slice: C<\@hash{@keys} = @those_scalar_refs>.

=item Can't msgrcv to read-only var

(F) The target of a msgrcv must be modifiable to be used as a receive
buffer.

=item Can't "next" outside a loop block

(F) A "next" statement was executed to reiterate the current block, but
there isn't a current block.  Note that an "if" or "else" block doesn't
count as a "loopish" block, as doesn't a block given to sort(), map() or
grep().  You can usually double the curlies to get the same effect
though, because the inner curlies will be considered a block that loops
once.  See L<perlfunc/next>.

=item Can't open %s: %s

(S inplace) The implicit opening of a file through use of the C<< <> >>
filehandle, either implicitly under the C<-n> or C<-p> command-line
switches, or explicitly, failed for the indicated reason.  Usually
this is because you don't have read permission for a file which
you named on the command line.

(F) You tried to call perl with the B<-e> switch, but F</dev/null> (or
your operating system's equivalent) could not be opened.

=item Can't open a reference

(W io) You tried to open a scalar reference for reading or writing,
using the 3-arg open() syntax:

    open FH, '>', $ref;

but your version of perl is compiled without perlio, and this form of
open is not supported.

=item Can't open bidirectional pipe

(W pipe) You tried to say C<open(CMD, "|cmd|")>, which is not supported.
You can try any of several modules in the Perl library to do this, such
as IPC::Open2.  Alternately, direct the pipe's output to a file using
">", and then read it in under a different file handle.

=item Can't open error file %s as stderr

(F) An error peculiar to VMS.  Perl does its own command line
redirection, and couldn't open the file specified after '2>' or '2>>' on
the command line for writing.

=item Can't open input file %s as stdin

(F) An error peculiar to VMS.  Perl does its own command line
redirection, and couldn't open the file specified after '<' on the
command line for reading.

=item Can't open output file %s as stdout

(F) An error peculiar to VMS.  Perl does its own command line
redirection, and couldn't open the file specified after '>' or '>>' on
the command line for writing.

=item Can't open output pipe (name: %s)

(P) An error peculiar to VMS.  Perl does its own command line
redirection, and couldn't open the pipe into which to send data destined
for stdout.

=item Can't open perl script "%s": %s

(F) The script you specified can't be opened for the indicated reason.

If you're debugging a script that uses #!, and normally relies on the
shell's $PATH search, the -S option causes perl to do that search, so
you don't have to type the path or C<`which $scriptname`>.

=item Can't read CRTL environ

(S) A warning peculiar to VMS.  Perl tried to read an element of %ENV
from the CRTL's internal environment array and discovered the array was
missing.  You need to figure out where your CRTL misplaced its environ
or define F<PERL_ENV_TABLES> (see L<perlvms>) so that environ is not
searched.

=item Can't redeclare "%s" in "%s"

(F) A "my", "our" or "state" declaration was found within another declaration,
such as C<my ($x, my($y), $z)> or C<our (my $x)>.

=item Can't "redo" outside a loop block

(F) A "redo" statement was executed to restart the current block, but
there isn't a current block.  Note that an "if" or "else" block doesn't
count as a "loopish" block, as doesn't a block given to sort(), map()
or grep().  You can usually double the curlies to get the same effect
though, because the inner curlies will be considered a block that
loops once.  See L<perlfunc/redo>.

=item Can't remove %s: %s, skipping file

(S inplace) You requested an inplace edit without creating a backup
file.  Perl was unable to remove the original file to replace it with
the modified file.  The file was left unmodified.

=item Can't rename in-place work file '%s' to '%s': %s

(F) When closed implicitly, the temporary file for in-place editing
couldn't be renamed to the original filename.

=item Can't rename %s to %s: %s, skipping file

(F) The rename done by the B<-i> switch failed for some reason,
probably because you don't have write permission to the directory.

=item Can't reopen input pipe (name: %s) in binary mode

(P) An error peculiar to VMS.  Perl thought stdin was a pipe, and tried
to reopen it to accept binary data.  Alas, it failed.

=item Can't represent character for Ox%X on this platform

(F) There is a hard limit to how big a character code point can be due
to the fundamental properties of UTF-8, especially on EBCDIC
platforms.  The given code point exceeds that.  The only work-around is
to not use such a large code point.

=item Can't reset %ENV on this system

(F) You called C<reset('E')> or similar, which tried to reset
all variables in the current package beginning with "E".  In
the main package, that includes %ENV.  Resetting %ENV is not
supported on some systems, notably VMS.

=item Can't resolve method "%s" overloading "%s" in package "%s"

(F)(P) Error resolving overloading specified by a method name (as
opposed to a subroutine reference): no such method callable via the
package.  If the method name is C<???>, this is an internal error.

=item Can't return %s from lvalue subroutine

(F) Perl detected an attempt to return illegal lvalues (such as
temporary or readonly values) from a subroutine used as an lvalue.  This
is not allowed.

=item Can't return outside a subroutine

(F) The return statement was executed in mainline code, that is, where
there was no subroutine call to return out of.  See L<perlsub>.

=item Can't return %s to lvalue scalar context

(F) You tried to return a complete array or hash from an lvalue
subroutine, but you called the subroutine in a way that made Perl
think you meant to return only one value.  You probably meant to
write parentheses around the call to the subroutine, which tell
Perl that the call should be in list context.

=item Can't take log of %g

(F) For ordinary real numbers, you can't take the logarithm of a
negative number or zero.  There's a Math::Complex package that comes
standard with Perl, though, if you really want to do that for the
negative numbers.

=item Can't take sqrt of %g

(F) For ordinary real numbers, you can't take the square root of a
negative number.  There's a Math::Complex package that comes standard
with Perl, though, if you really want to do that.

=item Can't undef active subroutine

(F) You can't undefine a routine that's currently running.  You can,
however, redefine it while it's running, and you can even undef the
redefined subroutine while the old routine is running.  Go figure.

=item Can't unweaken a nonreference

(F) You attempted to unweaken something that was not a reference.  Only
references can be unweakened.

=item Can't upgrade %s (%d) to %d

(P) The internal sv_upgrade routine adds "members" to an SV, making it
into a more specialized kind of SV.  The top several SV types are so
specialized, however, that they cannot be interconverted.  This message
indicates that such a conversion was attempted.

=item Can't use '%c' after -mname

(F) You tried to call perl with the B<-m> switch, but you put something
other than "=" after the module name.

=item Can't use a hash as a reference

(F) You tried to use a hash as a reference, as in
C<< %foo->{"bar"} >> or C<< %$ref->{"hello"} >>.  Versions of perl
<= 5.22.0 used to allow this syntax, but shouldn't
have.  This was deprecated in perl 5.6.1.

=item Can't use an array as a reference

(F) You tried to use an array as a reference, as in
C<< @foo->[23] >> or C<< @$ref->[99] >>.  Versions of perl <= 5.22.0
used to allow this syntax, but shouldn't have.  This
was deprecated in perl 5.6.1.

=item Can't use anonymous symbol table for method lookup

(F) The internal routine that does method lookup was handed a symbol
table that doesn't have a name.  Symbol tables can become anonymous
for example by undefining stashes: C<undef %Some::Package::>.

=item Can't use an undefined value as %s reference

(F) A value used as either a hard reference or a symbolic reference must
be a defined value.  This helps to delurk some insidious errors.

=item Can't use bareword ("%s") as %s ref while "strict refs" in use

(F) Only hard references are allowed by "strict refs".  Symbolic
references are disallowed.  See L<perlref>.

=item Can't use %! because Errno.pm is not available

(F) The first time the C<%!> hash is used, perl automatically loads the
Errno.pm module.  The Errno module is expected to tie the %! hash to
provide symbolic names for C<$!> errno values.

=item Can't use both '<' and '>' after type '%c' in %s

(F) A type cannot be forced to have both big-endian and little-endian
byte-order at the same time, so this combination of modifiers is not
allowed.  See L<perlfunc/pack>.

=item Can't use 'defined(@array)' (Maybe you should just omit the defined()?)

(F) defined() is not useful on arrays because it
checks for an undefined I<scalar> value.  If you want to see if the
array is empty, just use C<if (@array) { # not empty }> for example.

=item Can't use 'defined(%hash)' (Maybe you should just omit the defined()?)

(F) C<defined()> is not usually right on hashes.

Although C<defined %hash> is false on a plain not-yet-used hash, it
becomes true in several non-obvious circumstances, including iterators,
weak references, stash names, even remaining true after C<undef %hash>.
These things make C<defined %hash> fairly useless in practice, so it now
generates a fatal error.

If a check for non-empty is what you wanted then just put it in boolean
context (see L<perldata/Scalar values>):

    if (%hash) {
       # not empty
    }

If you had C<defined %Foo::Bar::QUUX> to check whether such a package
variable exists then that's never really been reliable, and isn't
a good way to enquire about the features of a package, or whether
it's loaded, etc.

=item Can't use %s for loop variable

(P) The parser got confused when trying to parse a C<foreach> loop.

=item Can't use global %s in %s

(F) You tried to declare a magical variable as a lexical variable.  This
is not allowed, because the magic can be tied to only one location
(namely the global variable) and it would be incredibly confusing to
have variables in your program that looked like magical variables but
weren't.

=item Can't use '%c' in a group with different byte-order in %s

(F) You attempted to force a different byte-order on a type
that is already inside a group with a byte-order modifier.
For example you cannot force little-endianness on a type that
is inside a big-endian group.

=item Can't use "my %s" in sort comparison

(F) The global variables $a and $b are reserved for sort comparisons.
You mentioned $a or $b in the same line as the <=> or cmp operator,
and the variable had earlier been declared as a lexical variable.
Either qualify the sort variable with the package name, or rename the
lexical variable.

=item Can't use %s ref as %s ref

(F) You've mixed up your reference types.  You have to dereference a
reference of the type needed.  You can use the ref() function to
test the type of the reference, if need be.

=item Can't use string ("%s") as %s ref while "strict refs" in use

=item Can't use string ("%s"...) as %s ref while "strict refs" in use

(F) You've told Perl to dereference a string, something which
C<use strict> blocks to prevent it happening accidentally.  See
L<perlref/"Symbolic references">.  This can be triggered by an C<@> or C<$>
in a double-quoted string immediately before interpolating a variable,
for example in C<"user @$twitter_id">, which says to treat the contents
of C<$twitter_id> as an array reference; use a C<\> to have a literal C<@>
symbol followed by the contents of C<$twitter_id>: C<"user \@$twitter_id">.

=item Can't use subscript on %s

(F) The compiler tried to interpret a bracketed expression as a
subscript.  But to the left of the brackets was an expression that
didn't look like a hash or array reference, or anything else subscriptable.

=item Can't use \%c to mean $%c in expression

(W syntax) In an ordinary expression, backslash is a unary operator that
creates a reference to its argument.  The use of backslash to indicate a
backreference to a matched substring is valid only as part of a regular
expression pattern.  Trying to do this in ordinary Perl code produces a
value that prints out looking like SCALAR(0xdecaf).  Use the $1 form
instead.

=item Can't weaken a nonreference

(F) You attempted to weaken something that was not a reference.  Only
references can be weakened.

=item Can't "when" outside a topicalizer

(F) You have used a when() block that is neither inside a C<foreach>
loop nor a C<given> block.  (Note that this error is issued on exit
from the C<when> block, so you won't get the error if the match fails,
or if you use an explicit C<continue>.)

=item Can't x= to read-only value

(F) You tried to repeat a constant value (often the undefined value)
with an assignment operator, which implies modifying the value itself.
Perhaps you need to copy the value to a temporary, and repeat that.

=item catch block requires a (VAR)

(F) You tried to use the C<try> and C<catch> syntax of C<use feature 'try'>
but did not include the error variable in the C<catch> block. The
parenthesized variable name is not optional, unlike in some other forms of
syntax you may be familiar with from CPAN modules or other languages.

The required syntax is

    try { ... }
    catch ($var) { ... }

=item Character following "\c" must be printable ASCII

(F) In C<\cI<X>>, I<X> must be a printable (non-control) ASCII character.

Note that ASCII characters that don't map to control characters are
discouraged, and will generate the warning (when enabled)
L</""\c%c" is more clearly written simply as "%s"">.

=item Character following \%c must be '{' or a single-character Unicode property name in regex; marked by <-- HERE in m/%s/

(F) (In the above the C<%c> is replaced by either C<p> or C<P>.)  You
specified something that isn't a legal Unicode property name.  Most
Unicode properties are specified by C<\p{...}>.  But if the name is a
single character one, the braces may be omitted.

=item Character in 'C' format wrapped in pack

(W pack) You said

    pack("C", $x)

where $x is either less than 0 or more than 255; the C<"C"> format is
only for encoding native operating system characters (ASCII, EBCDIC,
and so on) and not for Unicode characters, so Perl behaved as if you meant

    pack("C", $x & 255)

If you actually want to pack Unicode codepoints, use the C<"U"> format
instead.

=item Character in 'c' format wrapped in pack

(W pack) You said

    pack("c", $x)

where $x is either less than -128 or more than 127; the C<"c"> format
is only for encoding native operating system characters (ASCII, EBCDIC,
and so on) and not for Unicode characters, so Perl behaved as if you meant

    pack("c", $x & 255);

If you actually want to pack Unicode codepoints, use the C<"U"> format
instead.

=item Character in '%c' format wrapped in unpack

(W unpack) You tried something like

   unpack("H", "\x{2a1}")

where the format expects to process a byte (a character with a value
below 256), but a higher value was provided instead.  Perl uses the
value modulus 256 instead, as if you had provided:

   unpack("H", "\x{a1}")

=item Character in 'W' format wrapped in pack

(W pack) You said

    pack("U0W", $x)

where $x is either less than 0 or more than 255.  However, C<U0>-mode
expects all values to fall in the interval [0, 255], so Perl behaved
as if you meant:

    pack("U0W", $x & 255)

=item Character(s) in '%c' format wrapped in pack

(W pack) You tried something like

   pack("u", "\x{1f3}b")

where the format expects to process a sequence of bytes (character with a
value below 256), but some of the characters had a higher value.  Perl
uses the character values modulus 256 instead, as if you had provided:

   pack("u", "\x{f3}b")

=item Character(s) in '%c' format wrapped in unpack

(W unpack) You tried something like

   unpack("s", "\x{1f3}b")

where the format expects to process a sequence of bytes (character with a
value below 256), but some of the characters had a higher value.  Perl
uses the character values modulus 256 instead, as if you had provided:

   unpack("s", "\x{f3}b")

=item charnames alias definitions may not contain a sequence of multiple
spaces; marked by S<<-- HERE> in %s

(F) You defined a character name which had multiple space characters
in a row.  Change them to single spaces.  Usually these names are
defined in the C<:alias> import argument to C<use charnames>, but they
could be defined by a translator installed into C<$^H{charnames}>.  See
L<charnames/CUSTOM ALIASES>.

=item chdir() on unopened filehandle %s

(W unopened) You tried chdir() on a filehandle that was never opened.

=item "\c%c" is more clearly written simply as "%s"

(W syntax) The C<\cI<X>> construct is intended to be a way to specify
non-printable characters.  You used it for a printable one, which
is better written as simply itself, perhaps preceded by a backslash
for non-word characters.  Doing it the way you did is not portable
between ASCII and EBCDIC platforms.

=item Class already has a superclass, cannot add another

(F) You attempted to specify a second superclass for a C<class> by using
the C<:isa> attribute, when one is already specified.  Unlike classes
whose instances are created with C<bless>, classes created via the
C<class> keyword cannot have more than one superclass.

=item Class attribute %s requires a value

(F) You specified an attribute for a class that would require a value to
be passed in parentheses, but did not provide one.  Remember that
whitespace is B<not> permitted between the attribute name and its value;
you must write this as

    class Example::Class :attr(VALUE) ...

=item class is experimental

(S experimental::class) This warning is emitted if you use the C<class>
keyword of C<use feature 'class'>.  This keyword is currently
experimental and its behaviour may change in future releases of Perl.

=item Class :isa attribute requires a class but "%s" is not one

(F) When creating a subclass using the C<class> C<:isa> attribute, the
named superclass must also be a real class created using the C<class>
keyword.

=item Cloning substitution context is unimplemented

(F) Creating a new thread inside the C<s///> operator is not supported.

=item closedir() attempted on invalid dirhandle %s

(W io) The dirhandle you tried to close is either closed or not really
a dirhandle.  Check your control flow.

=item close() on unopened filehandle %s

(W unopened) You tried to close a filehandle that was never opened.

=item Closure prototype called

(F) If a closure has attributes, the subroutine passed to an attribute
handler is the prototype that is cloned when a new closure is created.
This subroutine cannot be called.

=item \C no longer supported in regex; marked by S<<-- HERE> in m/%s/

(F) The \C character class used to allow a match of single byte
within a multi-byte utf-8 character, but was removed in v5.24 as
it broke encapsulation and its implementation was extremely buggy.
If you really need to process the individual bytes, you probably
want to convert your string to one where each underlying byte is
stored as a character, with utf8::encode().

=item Code missing after '/'

(F) You had a (sub-)template that ends with a '/'.  There must be
another template code following the slash.  See L<perlfunc/pack>.

=item Code point 0x%X is not Unicode, and not portable

(S non_unicode portable) You had a code point that has never been in any
standard, so it is likely that languages other than Perl will NOT
understand it.  This code point also will not fit in a 32-bit word on
ASCII platforms and therefore is non-portable between systems.

At one time, it was legal in some standards to have code points up to
0x7FFF_FFFF, but not higher, and this code point is higher.

Acceptance of these code points is a Perl extension, and you should
expect that nothing other than Perl can handle them; Perl itself on
EBCDIC platforms before v5.24 does not handle them.

Perl also makes no guarantees that the representation of these code
points won't change at some point in the future, say when machines
become available that have larger than a 64-bit word.  At that time,
files containing any of these, written by an older Perl might require
conversion before being readable by a newer Perl.

=item Code point 0x%X is not Unicode, may not be portable

(S non_unicode) You had a code point above the Unicode maximum
of U+10FFFF.

Perl allows strings to contain a superset of Unicode code points, but
these may not be accepted by other languages/systems.  Further, even if
these languages/systems accept these large code points, they may have
chosen a different representation for them than the UTF-8-like one that
Perl has, which would mean files are not exchangeable between them and
Perl.

On EBCDIC platforms, code points above 0x3FFF_FFFF have a different
representation in Perl v5.24 than before, so any file containing these
that was written before that version will require conversion before
being readable by a later Perl.

=item %s: Command not found

(A) You've accidentally run your script through B<csh> or another shell
instead of Perl.  Check the #! line, or manually feed your script into
Perl yourself.  The #! line at the top of your file could look like

  #!/usr/bin/perl

=item %s: command not found

(A) You've accidentally run your script through B<bash> or another shell
instead of Perl.  Check the #! line, or manually feed your script into
Perl yourself.  The #! line at the top of your file could look like

  #!/usr/bin/perl

=item %s: command not found: %s

(A) You've accidentally run your script through B<zsh> or another shell
instead of Perl.  Check the #! line, or manually feed your script into
Perl yourself.  The #! line at the top of your file could look like

  #!/usr/bin/perl

=item Compilation failed in require

(F) Perl could not compile a file specified in a C<require> statement.
Perl uses this generic message when none of the errors that it
encountered were severe enough to halt compilation immediately.

=item connect() on closed socket %s

(W closed) You tried to do a connect on a closed socket.  Did you forget
to check the return value of your socket() call?  See
L<perlfunc/connect>.

=item Constant(%s): Call to &{$^H{%s}} did not return a defined value

(F) The subroutine registered to handle constant overloading
(see L<overload>) or a custom charnames handler (see
L<charnames/CUSTOM TRANSLATORS>) returned an undefined value.

=item Constant(%s): $^H{%s} is not defined

(F) The parser found inconsistencies while attempting to define an
overloaded constant.  Perhaps you forgot to load the corresponding
L<overload> pragma?

=item Constant is not %s reference

(F) A constant value (perhaps declared using the C<use constant> pragma)
is being dereferenced, but it amounts to the wrong type of reference.
The message indicates the type of reference that was expected.  This
usually indicates a syntax error in dereferencing the constant value.
See L<perlsub/"Constant Functions"> and L<constant>.

=item Constants from lexical variables potentially modified elsewhere are no longer permitted

(F) You wrote something like

    my $var;
    $sub = sub () { $var };

but $var is referenced elsewhere and could be modified after the C<sub>
expression is evaluated.  Either it is explicitly modified elsewhere
(C<$var = 3>) or it is passed to a subroutine or to an operator like
C<printf> or C<map>, which may or may not modify the variable.

Traditionally, Perl has captured the value of the variable at that
point and turned the subroutine into a constant eligible for inlining.
In those cases where the variable can be modified elsewhere, this
breaks the behavior of closures, in which the subroutine captures
the variable itself, rather than its value, so future changes to the
variable are reflected in the subroutine's return value.

This usage was deprecated, and as of Perl 5.32 is no longer allowed,
making it possible to change the behavior in the future.

If you intended for the subroutine to be eligible for inlining, then
make sure the variable is not referenced elsewhere, possibly by
copying it:

    my $var2 = $var;
    $sub = sub () { $var2 };

If you do want this subroutine to be a closure that reflects future
changes to the variable that it closes over, add an explicit C<return>:

    my $var;
    $sub = sub () { return $var };

=item Constant subroutine %s redefined

(W redefine)(S) You redefined a subroutine which had previously
been eligible for inlining.  See L<perlsub/"Constant Functions">
for commentary and workarounds.

=item Constant subroutine %s undefined

(W misc) You undefined a subroutine which had previously been eligible
for inlining.  See L<perlsub/"Constant Functions"> for commentary and
workarounds.

=item Constant(%s) unknown

(F) The parser found inconsistencies either while attempting
to define an overloaded constant, or when trying to find the
character name specified in the C<\N{...}> escape.  Perhaps you
forgot to load the corresponding L<overload> pragma?

=item :const is experimental

(S experimental::const_attr) The "const" attribute is experimental.
If you want to use the feature, disable the warning with C<no warnings
'experimental::const_attr'>, but know that in doing so you are taking
the risk that your code may break in a future Perl version.

=item :const is not permitted on named subroutines

(F) The "const" attribute causes an anonymous subroutine to be run and
its value captured at the time that it is cloned.  Named subroutines are
not cloned like this, so the attribute does not make sense on them.

=item Copy method did not return a reference

(F) The method which overloads "=" is buggy.  See
L<overload/Copy Constructor>.

=item &CORE::%s cannot be called directly

(F) You tried to call a subroutine in the C<CORE::> namespace
with C<&foo> syntax or through a reference.  Some subroutines
in this package cannot yet be called that way, but must be
called as barewords.  Something like this will work:

    BEGIN { *shove = \&CORE::push; }
    shove @array, 1,2,3; # pushes on to @array

=item CORE::%s is not a keyword

(F) The CORE:: namespace is reserved for Perl keywords.

=item Corrupted regexp opcode %d > %d

(P) This is either an error in Perl, or, if you're using
one, your L<custom regular expression engine|perlreapi>.  If not the
latter, report the problem to L<https://github.com/Perl/perl5/issues/new/choose>.

=item corrupted regexp pointers

(P) The regular expression engine got confused by what the regular
expression compiler gave it.

=item corrupted regexp program

(P) The regular expression engine got passed a regexp program without a
valid magic number.

=item Corrupt malloc ptr 0x%x at 0x%x

(P) The malloc package that comes with Perl had an internal failure.

=item Count after length/code in unpack

(F) You had an unpack template indicating a counted-length string, but
you have also specified an explicit size for the string.  See
L<perlfunc/pack>.

=item Declaring references is experimental

(S experimental::declared_refs) This warning is emitted if you use
a reference constructor on the right-hand side of C<my>, C<state>, C<our>, or
C<local>.  Simply suppress the warning if you want to use the feature, but
know that in doing so you are taking the risk of using an experimental
feature which may change or be removed in a future Perl version:

    no warnings "experimental::declared_refs";
    use feature "declared_refs";
    $fooref = my \$foo;

=for comment
The following are used in lib/diagnostics.t for testing two =items that
share the same description.  Changes here need to be propagated to there

=item Deep recursion on anonymous subroutine

=item Deep recursion on subroutine "%s"

(W recursion) This subroutine has called itself (directly or indirectly)
100 times more than it has returned.  This probably indicates an
infinite recursion, unless you're writing strange benchmark programs, in
which case it indicates something else.

This threshold can be changed from 100, by recompiling the F<perl> binary,
setting the C pre-processor macro C<PERL_SUB_DEPTH_WARN> to the desired value.

=item (?(DEFINE)....) does not allow branches in regex; marked by
S<<-- HERE> in m/%s/

(F) You used something like C<(?(DEFINE)...|..)> which is illegal.  The
most likely cause of this error is that you left out a parenthesis inside
of the C<....> part.

The S<<-- HERE> shows whereabouts in the regular expression the problem was
discovered.

=item %s defines neither package nor VERSION--version check failed

(F) You said something like "use Module 42" but in the Module file
there are neither package declarations nor a C<$VERSION>.

=item delete argument is not a HASH or ARRAY element or slice

(F) The argument to C<delete> must be either a hash or array element,
such as:

    $foo{$bar}
    $ref->{"susie"}[12]

or a hash or array slice, such as:

    @foo[$bar, $baz, $xyzzy]
    $ref->[12]->@{"susie", "queue"}

or a hash key/value or array index/value slice, such as:

    %foo[$bar, $baz, $xyzzy]
    $ref->[12]->%{"susie", "queue"}

=item Delimiter for here document is too long

(F) In a here document construct like C<<<FOO>, the label C<FOO> is too
long for Perl to handle.  You have to be seriously twisted to write code
that triggers this error.

=item DESTROY created new reference to dead object '%s'

(F) A DESTROY() method created a new reference to the object which is
just being DESTROYed.  Perl is confused, and prefers to abort rather
than to create a dangling reference.

=item Did not produce a valid header

See L</500 Server error>.

=item %s did not return a true value

(F) A required (or used) file must return a true value to indicate that
it compiled correctly and ran its initialization code correctly.  It's
traditional to end such a file with a "1;", though any true value would
do.  See L<perlfunc/require>.

=item (Did you mean &%s instead?)

(W misc) You probably referred to an imported subroutine &FOO as $FOO or
some such.

=item (Did you mean "local" instead of "our"?)

(W shadow) Remember that "our" does not localize the declared global
variable.  You have declared it again in the same lexical scope, which
seems superfluous.

=item (Did you mean $ or @ instead of %?)

(W) You probably said %hash{$key} when you meant $hash{$key} or
@hash{@keys}.  On the other hand, maybe you just meant %hash and got
carried away.

=item Died

(F) You passed die() an empty string (the equivalent of C<die "">) or
you called it with no args and C<$@> was empty.

=item Document contains no data

See L</500 Server error>.

=item %s does not define %s::VERSION--version check failed

(F) You said something like "use Module 42" but the Module did not
define a C<$VERSION>.

=item '/' does not take a repeat count in %s

(F) You cannot put a repeat count of any kind right after the '/' code.
See L<perlfunc/pack>.

=item do "%s" failed, '.' is no longer in @INC; did you mean do "./%s"?

(D deprecated::dot_in_inc) Previously C< do "somefile"; > would search
the current directory for the specified file. Since perl v5.26.0, F<.>
has been removed from C<@INC> by default, so this is no longer true. To
search the current directory (and only the current directory) you can
write C< do "./somefile"; >.

=item Don't know how to get file name

(P) C<PerlIO_getname>, a perl internal I/O function specific to VMS, was
somehow called on another platform.  This should not happen.

=item Don't know how to handle magic of type \%o

(P) The internal handling of magical variables has been cursed.

=item Downgrading a use VERSION declaration to below v5.11 is deprecated

(S deprecated::version_downgrade) This warning is emitted on a
C<use VERSION> statement that requests a version below v5.11 (when the
effects of C<use strict> would be disabled), after a previous
declaration of one having a larger number (which would have enabled
these effects). Because of a change to the way that C<use VERSION>
interacts with the strictness flags, this is no longer supported.

=item (Do you need to predeclare %s?)

(S syntax) This is an educated guess made in conjunction with the message
"%s found where operator expected".  It often means a subroutine or module
name is being referenced that hasn't been declared yet.  This may be
because of ordering problems in your file, or because of a missing
"sub", "package", "require", or "use" statement.  If you're referencing
something that isn't defined yet, you don't actually have to define the
subroutine or package before the current location.  You can use an empty
"sub foo;" or "package FOO;" to enter a "forward" declaration.

=item dump() must be written as CORE::dump() as of Perl 5.30

(F) You used the obsolete C<dump()> built-in function.  That was deprecated in
Perl 5.8.0.  As of Perl 5.30 it must be written in fully qualified format:
C<CORE::dump()>.

See L<perlfunc/dump>.

=item dump is not supported

(F) Your machine doesn't support dump/undump.

=item Duplicate free() ignored

(S malloc) An internal routine called free() on something that had
already been freed.

=item Duplicate modifier '%c' after '%c' in %s

(W unpack) You have applied the same modifier more than once after a
type in a pack template.  See L<perlfunc/pack>.

=item each on anonymous %s will always start from the beginning

(W syntax) You called L<each|perlfunc/each> on an anonymous hash or
array.  Since a new hash or array is created each time, each() will
restart iterating over your hash or array every time.

=item elseif should be elsif

(S syntax) There is no keyword "elseif" in Perl because Larry thinks
it's ugly.  Your code will be interpreted as an attempt to call a method
named "elseif" for the class returned by the following block.  This is
unlikely to be what you want.

=item Empty \%c in regex; marked by S<<-- HERE> in m/%s/

=item Empty \%c{}

=item Empty \%c{} in regex; marked by S<<-- HERE> in m/%s/

(F) You used something like C<\b{}>, C<\B{}>, C<\o{}>, C<\p>, C<\P>, or
C<\x> without specifying anything for it to operate on.

Unfortunately, for backwards compatibility reasons, an empty C<\x> is
legal outside S<C<use re 'strict'>> and expands to a NUL character.

=item Empty (?) without any modifiers in regex; marked by <-- HERE in m/%s/

(W regexp) (only under C<S<use re 'strict'>>)
C<(?)> does nothing, so perhaps this is a typo.

=item ${^ENCODING} is no longer supported

(F) The special variable C<${^ENCODING}>, formerly used to implement
the C<encoding> pragma, is no longer supported as of Perl 5.26.0.

Setting it to anything other than C<undef> is a fatal error as of Perl
5.28.

=item ${^HOOK}{%s} may only be a CODE reference or undef

(F) You attempted to assign something other than undef or a CODE ref to
C<%{^HOOK}>. Hooks may only be CODE refs. See L<perlvar/%{^HOOK}> for
details.

=item Attempt to set unknown hook '%s' in %{^HOOK}

(F) You attempted to assign something other than undef or a CODE ref to
C<%{^HOOK}>. Hooks may only be CODE refs. See L<perlvar/%{^HOOK}> for
details.

=item entering effective %s failed

(F) While under the C<use filetest> pragma, switching the real and
effective uids or gids failed.

=item %ENV is aliased to %s

(F) You're running under taint mode, and the C<%ENV> variable has been
aliased to another hash, so it doesn't reflect anymore the state of the
program's environment.  This is potentially insecure.

=item Error converting file specification %s

(F) An error peculiar to VMS.  Because Perl may have to deal with file
specifications in either VMS or Unix syntax, it converts them to a
single form when it must operate on them directly.  Either you've passed
an invalid file specification to Perl, or you've found a case the
conversion routines don't handle.  Drat.

=item Error %s in expansion of %s

(F) An error was encountered in handling a user-defined property
(L<perlunicode/User-Defined Character Properties>).  These are
programmer written subroutines, hence subject to errors that may
prevent them from compiling or running.  The calls to these subs are
C<eval>'d, and if there is a failure, this message is raised, using the
contents of C<$@> from the failed C<eval>.

Another possibility is that tainted data was encountered somewhere in
the chain of expanding the property.  If so, the message wording will
indicate that this is the problem.  See L</Insecure user-defined
property %s>.

=item Eval-group in insecure regular expression

(F) Perl detected tainted data when trying to compile a regular
expression that contains the C<(?{ ... })> zero-width assertion, which
is unsafe.  See L<perlre/(?{ code })>, and L<perlsec>.

=item Eval-group not allowed at runtime, use re 'eval' in regex m/%s/

(F) Perl tried to compile a regular expression containing the
C<(?{ ... })> zero-width assertion at run time, as it would when the
pattern contains interpolated values.  Since that is a security risk,
it is not allowed.  If you insist, you may still do this by using the
C<re 'eval'> pragma or by explicitly building the pattern from an
interpolated string at run time and using that in an eval().  See
L<perlre/(?{ code })>.

=item Eval-group not allowed, use re 'eval' in regex m/%s/

(F) A regular expression contained the C<(?{ ... })> zero-width
assertion, but that construct is only allowed when the C<use re 'eval'>
pragma is in effect.  See L<perlre/(?{ code })>.

=item EVAL without pos change exceeded limit in regex; marked by
S<<-- HERE> in m/%s/

(F) You used a pattern that nested too many EVAL calls without consuming
any text.  Restructure the pattern so that text is consumed.

The S<<-- HERE> shows whereabouts in the regular expression the problem was
discovered.

=item Excessively long <> operator

(F) The contents of a <> operator may not exceed the maximum size of a
Perl identifier.  If you're just trying to glob a long list of
filenames, try using the glob() operator, or put the filenames into a
variable and glob that.

=item exec? I'm not *that* kind of operating system

(F) The C<exec> function is not implemented on some systems, e.g.
Catamount. See L<perlport>.

=item %sExecution of %s aborted due to compilation errors.

(F) The final summary message when a Perl compilation fails.

=item Execution of %s aborted due to compilation errors.

(F) The final summary message when a Perl compilation fails.

=item exists argument is not a HASH or ARRAY element or a subroutine

(F) The argument to C<exists> must be a hash or array element or a
subroutine with an ampersand, such as:

    $foo{$bar}
    $ref->{"susie"}[12]
    &do_something

=item exists argument is not a subroutine name

(F) The argument to C<exists> for C<exists &sub> must be a subroutine name,
and not a subroutine call.  C<exists &sub()> will generate this error.

=item Exiting eval via %s

(W exiting) You are exiting an eval by unconventional means, such as a
goto, or a loop control statement.

=item Exiting format via %s

(W exiting) You are exiting a format by unconventional means, such as a
goto, or a loop control statement.

=item Exiting pseudo-block via %s

(W exiting) You are exiting a rather special block construct (like a
sort block or subroutine) by unconventional means, such as a goto, or a
loop control statement.  See L<perlfunc/sort>.

=item Exiting subroutine via %s

(W exiting) You are exiting a subroutine by unconventional means, such
as a goto, or a loop control statement.

=item Exiting substitution via %s

(W exiting) You are exiting a substitution by unconventional means, such
as a return, a goto, or a loop control statement.

=item Expected %s reference in export_lexically

(F) The type of a reference given to L<builtin/export_lexically> did not
match the sigil of the preceding name, or the value was not a reference at
all. 

=item Expecting close bracket in regex; marked by S<<-- HERE> in m/%s/

(F) You wrote something like

 (?13

to denote a capturing group of the form
L<C<(?I<PARNO>)>|perlre/(?PARNO) (?-PARNO) (?+PARNO) (?R) (?0)>,
but omitted the C<")">.

=item Expecting interpolated extended charclass in regex; marked by <--
HERE in m/%s/

(F) It looked like you were attempting to interpolate an
already-compiled extended character class, like so:

 my $thai_or_lao = qr/(?[ \p{Thai} + \p{Lao} ])/;
 ...
 qr/(?[ \p{Digit} & $thai_or_lao ])/;

But the marked code isn't syntactically correct to be such an
interpolated class.

=item Experimental aliasing via reference not enabled

(F) To do aliasing via references, you must first enable the feature:

    no warnings "experimental::refaliasing";
    use feature "refaliasing";
    \$x = \$y;

=item Experimental %s on scalar is now forbidden

(F) An experimental feature added in Perl 5.14 allowed C<each>, C<keys>,
C<push>, C<pop>, C<shift>, C<splice>, C<unshift>, and C<values> to be called with a
scalar argument.  This experiment is considered unsuccessful, and
has been removed.  The C<postderef> feature may meet your needs better.

=item Experimental subroutine signatures not enabled

(F) To use subroutine signatures, you must first enable them:

    use feature "signatures";
    sub foo ($left, $right) { ... }

=item Explicit blessing to '' (assuming package main)

(W misc) You are blessing a reference to a zero length string.  This has
the effect of blessing the reference into the package main.  This is
usually not what you want.  Consider providing a default target package,
e.g. bless($ref, $p || 'MyPackage');

=item export_lexically can only be called at compile time

(F) L<builtin/export_lexically> was called at runtime.  Because it creates
new names in the lexical scope currently being compiled, it can only be
called from code inside C<BEGIN> block in that scope.

=item %s: Expression syntax

(A) You've accidentally run your script through B<csh> instead of Perl.
Check the #! line, or manually feed your script into Perl yourself.

=item %s failed--call queue aborted

(F) An untrapped exception was raised while executing a UNITCHECK,
CHECK, INIT, or END subroutine.  Processing of the remainder of the
queue of such routines has been prematurely ended.

=item Failed to close in-place work file %s: %s

(F) Closing an output file from in-place editing, as with the C<-i>
command-line switch, failed.

=item False [] range "%s" in regex; marked by S<<-- HERE> in m/%s/

(W regexp)(F) A character class range must start and end at a literal
character, not another character class like C<\d> or C<[:alpha:]>.  The "-"
in your false range is interpreted as a literal "-".  In a C<(?[...])>
construct, this is an error, rather than a warning.  Consider quoting
the "-", "\-".  The S<<-- HERE> shows whereabouts in the regular expression
the problem was discovered.  See L<perlre>.

=item Fatal VMS error (status=%d) at %s, line %d

(P) An error peculiar to VMS.  Something untoward happened in a VMS
system service or RTL routine; Perl's exit status should provide more
details.  The filename in "at %s" and the line number in "line %d" tell
you which section of the Perl source code is distressed.

=item fcntl is not implemented

(F) Your machine apparently doesn't implement fcntl().  What is this, a
PDP-11 or something?

=item FETCHSIZE returned a negative value

(F) A tied array claimed to have a negative number of elements, which
is not possible.

=item Field already has a parameter name, cannot add another

(F) A field may have at most one application of the C<:param> attribute to
assign a parameter name to it; once applied a second one is not allowed.

=item Field attribute %s requires a value

(F) You specified an attribute for a field that would require a value to
be passed in parentheses, but did not provide one.  Remember that
whitespace is B<not> permitted between the attribute name and its value;
you must write this as

    field $var :attr(VALUE) ...

=item field is experimental

(S experimental::class) This warning is emitted if you use the C<field>
keyword of C<use feature 'class'>.  This keyword is currently
experimental and its behaviour may change in future releases of Perl.

=item Field %s is not accessible outside a method

(F) An attempt was made to access a field variable of a class from code
that does not appear inside the body of a C<method> subroutine.  This is not
permitted, as only methods will have access to the fields of an instance.

=item Field %s of "%s" is not accessible in a method of "%s"

(F) An attempt was made to access a field variable of a class, from a
method of another class nested inside the one that actually defined it.
This is not permitted, as only methods defined by a given class are
permitted to access fields of that class.

=item Field too wide in 'u' format in pack

(W pack) Each line in an uuencoded string starts with a length indicator
which can't encode values above 63.  So there is no point in asking for
a line length bigger than that.  Perl will behave as if you specified
C<u63> as the format.

=item Filehandle %s opened only for input

(W io) You tried to write on a read-only filehandle.  If you intended
it to be a read-write filehandle, you needed to open it with "+<" or
"+>" or "+>>" instead of with "<" or nothing.  If you intended only to
write the file, use ">" or ">>".  See L<perlfunc/open>.

=item Filehandle %s opened only for output

(W io) You tried to read from a filehandle opened only for writing, If
you intended it to be a read/write filehandle, you needed to open it
with "+<" or "+>" or "+>>" instead of with ">".  If you intended only to
read from the file, use "<".  See L<perlfunc/open>.  Another possibility
is that you attempted to open filedescriptor 0 (also known as STDIN) for
output (maybe you closed STDIN earlier?).

=item Filehandle %s reopened as %s only for input

(W io) You opened for reading a filehandle that got the same filehandle id
as STDOUT or STDERR.  This occurred because you closed STDOUT or STDERR
previously.

=item Filehandle STDIN reopened as %s only for output

(W io) You opened for writing a filehandle that got the same filehandle id
as STDIN.  This occurred because you closed STDIN previously.

=item Filehandle STD%s reopened as %s only for input

(W io) You opened for reading a filehandle that got the same filehandle id
as STDOUT or STDERR.  This occurred because you closed the handle previously.

=item Final $ should be \$ or $name

(F) You must now decide whether the final $ in a string was meant to be
a literal dollar sign, or was meant to introduce a variable name that
happens to be missing.  So you have to put either the backslash or the
name.

=item defer is experimental

(S experimental::defer) The C<defer> block modifier is experimental. If you
want to use the feature, disable the warning with
C<no warnings 'experimental::defer'>, but know that in doing so you are taking
the risk that your code may break in a future Perl version.

=item flock() on closed filehandle %s

(W closed) The filehandle you're attempting to flock() got itself closed
some time before now.  Check your control flow.  flock() operates on
filehandles.  Are you attempting to call flock() on a dirhandle by the
same name?

=item for my (...) is experimental

(S experimental::for_list) This warning is emitted if you use C<for> to
iterate multiple values at a time. This syntax is currently experimental
and its behaviour may change in future releases of Perl.

=item Format not terminated

(F) A format must be terminated by a line with a solitary dot.  Perl got
to the end of your file without finding such a line.

=item Format %s redefined

(W redefine) You redefined a format.  To suppress this warning, say

    {
	no warnings 'redefine';
	eval "format NAME =...";
    }

=item Found = in conditional, should be ==

(W syntax) You said

    if ($foo = 123)

when you meant

    if ($foo == 123)

(or something like that).

=item %s found where operator expected

(S syntax) The Perl lexer knows whether to expect a term or an operator.
If it sees what it knows to be a term when it was expecting to see an
operator, it gives you this warning.  Usually it indicates that an
operator or delimiter was omitted, such as a semicolon.

=item gdbm store returned %d, errno %d, key "%s"

(S) A warning from the GDBM_File extension that a store failed.

=item gethostent not implemented

(F) Your C library apparently doesn't implement gethostent(), probably
because if it did, it'd feel morally obligated to return every hostname
on the Internet.

=item get%sname() on closed socket %s

(W closed) You tried to get a socket or peer socket name on a closed
socket.  Did you forget to check the return value of your socket() call?

=item getpwnam returned invalid UIC %#o for user "%s"

(S) A warning peculiar to VMS.  The call to C<sys$getuai> underlying the
C<getpwnam> operator returned an invalid UIC.

=item getsockopt() on closed socket %s

(W closed) You tried to get a socket option on a closed socket.  Did you
forget to check the return value of your socket() call?  See
L<perlfunc/getsockopt>.

=item given is deprecated

(D deprecated::smartmatch) C<given> depends on smartmatch, which is
deprecated. It will be removed in Perl 5.42. See the explanation under
L<perlsyn/Experimental Details on given and when>.

=item Global symbol "%s" requires explicit package name (did you forget to
declare "my %s"?)

(F) You've said "use strict" or "use strict vars", which indicates 
that all variables must either be lexically scoped (using "my" or "state"), 
declared beforehand using "our", or explicitly qualified to say 
which package the global variable is in (using "::").

=item glob failed (%s)

(S glob) Something went wrong with the external program(s) used
for C<glob> and C<< <*.c> >>.  Usually, this means that you supplied a C<glob>
pattern that caused the external program to fail and exit with a
nonzero status.  If the message indicates that the abnormal exit
resulted in a coredump, this may also mean that your csh (C shell)
is broken.  If so, you should change all of the csh-related variables
in config.sh:  If you have tcsh, make the variables refer to it as
if it were csh (e.g. C<full_csh='/usr/bin/tcsh'>); otherwise, make them
all empty (except that C<d_csh> should be C<'undef'>) so that Perl will
think csh is missing.  In either case, after editing config.sh, run
C<./Configure -S> and rebuild Perl.

=item Glob not terminated

(F) The lexer saw a left angle bracket in a place where it was expecting
a term, so it's looking for the corresponding right angle bracket, and
not finding it.  Chances are you left some needed parentheses out
earlier in the line, and you really meant a "less than".

=item gmtime(%f) failed

(W overflow) You called C<gmtime> with a number that it could not handle:
too large, too small, or NaN.  The returned value is C<undef>.

=item gmtime(%f) too large

(W overflow) You called C<gmtime> with a number that was larger than
it can reliably handle and C<gmtime> probably returned the wrong
date.  This warning is also triggered with NaN (the special
not-a-number value).

=item gmtime(%f) too small

(W overflow) You called C<gmtime> with a number that was smaller than
it can reliably handle and C<gmtime> probably returned the wrong date.

=item Got an error from DosAllocMem

(P) An error peculiar to OS/2.  Most probably you're using an obsolete
version of Perl, and this should not happen anyway.

=item goto must have label

(F) Unlike with "next" or "last", you're not allowed to goto an
unspecified destination.  See L<perlfunc/goto>.

=item Goto undefined subroutine%s

(F) You tried to call a subroutine with C<goto &sub> syntax, but
the indicated subroutine hasn't been defined, or if it was, it
has since been undefined.

=item Group name must start with a non-digit word character in regex; marked by 
S<<-- HERE> in m/%s/

(F) Group names must follow the rules for perl identifiers, meaning
they must start with a non-digit word character.  A common cause of
this error is using (?&0) instead of (?0).  See L<perlre>.

=item ()-group starts with a count

(F) A ()-group started with a count.  A count is supposed to follow
something: a template character or a ()-group.  See L<perlfunc/pack>.

=item %s had compilation errors.

(F) The final summary message when a C<perl -c> fails.

=item Had to create %s unexpectedly

(S internal) A routine asked for a symbol from a symbol table that ought
to have existed already, but for some reason it didn't, and had to be
created on an emergency basis to prevent a core dump.

=item %s has too many errors

(F) The parser has given up trying to parse the program after 10 errors.
Further error messages would likely be uninformative.

=item Hexadecimal float: exponent overflow

(W overflow) The hexadecimal floating point has a larger exponent
than the floating point supports.

=item Hexadecimal float: exponent underflow

(W overflow) The hexadecimal floating point has a smaller exponent
than the floating point supports.  With the IEEE 754 floating point,
this may also mean that the subnormals (formerly known as denormals)
are being used, which may or may not be an error.

=item Hexadecimal float: internal error (%s)

(F) Something went horribly bad in hexadecimal float handling.

=item Hexadecimal float: mantissa overflow

(W overflow) The hexadecimal floating point literal had more bits in
the mantissa (the part between the 0x and the exponent, also known as
the fraction or the significand) than the floating point supports.

=item Hexadecimal float: precision loss

(W overflow) The hexadecimal floating point had internally more
digits than could be output.  This can be caused by unsupported
long double formats, or by 64-bit integers not being available
(needed to retrieve the digits under some configurations).

=item Hexadecimal float: unsupported long double format

(F) You have configured Perl to use long doubles but
the internals of the long double format are unknown;
therefore the hexadecimal float output is impossible.

=item Hexadecimal number > 0xffffffff non-portable

(W portable) The hexadecimal number you specified is larger than 2**32-1
(4294967295) and therefore non-portable between systems.  See
L<perlport> for more on portability concerns.

=item Identifier too long

(F) Perl limits identifiers (names for variables, functions, etc.) to
about 250 characters for simple names, and somewhat more for compound
names (like C<$A::B>).  You've exceeded Perl's limits.  Future versions
of Perl are likely to eliminate these arbitrary limitations.

=item Ignoring zero length \N{} in character class in regex; marked by
S<<-- HERE> in m/%s/

(W regexp) Named Unicode character escapes (C<\N{...}>) may return a
zero-length sequence.  When such an escape is used in a character
class its behavior is not well defined.  Check that the correct
escape has been used, and the correct charname handler is in scope.

=item Illegal %s digit '%c' ignored

(W digit) Here C<%s> is one of "binary", "octal", or "hex".
You may have tried to use a digit other than one that is legal for the
given type, such as only 0 and 1 for binary.  For octals, this is raised
only if the illegal character is an '8' or '9'.  For hex, 'A' - 'F' and
'a' - 'f' are legal.
Interpretation of the number stopped just before the offending digit or
character.

=item Illegal binary digit '%c'

(F) You used a digit other than 0 or 1 in a binary number.

=item Illegal character after '_' in prototype for %s : %s

(W illegalproto) An illegal character was found in a prototype
declaration.  The '_' in a prototype must be followed by a ';',
indicating the rest of the parameters are optional, or one of '@'
or '%', since those two will accept 0 or more final parameters.

=item Illegal character \%o (carriage return)

(F) Perl normally treats carriage returns in the program text as
it would any other whitespace, which means you should never see
this error when Perl was built using standard options.  For some
reason, your version of Perl appears to have been built without
this support.  Talk to your Perl administrator.

=item Illegal character following sigil in a subroutine signature

(F) A parameter in a subroutine signature contained an unexpected character
following the C<$>, C<@> or C<%> sigil character.  Normally the sigil
should be followed by the variable name or C<=> etc.  Perhaps you are
trying use a prototype while in the scope of C<use feature 'signatures'>?
For example:

    sub foo ($$) {}            # legal - a prototype

    use feature 'signatures;
    sub foo ($$) {}            # illegal - was expecting a signature
    sub foo ($a, $b)
            :prototype($$) {}  # legal


=item Illegal character in prototype for %s : %s

(W illegalproto) An illegal character was found in a prototype declaration.
Legal characters in prototypes are $, @, %, *, ;, [, ], &, \, and +.
Perhaps you were trying to write a subroutine signature but didn't enable
that feature first (C<use feature 'signatures'>), so your signature was
instead interpreted as a bad prototype.

=item Illegal declaration of anonymous subroutine

(F) When using the C<sub> keyword to construct an anonymous subroutine,
you must always specify a block of code.  See L<perlsub>.

=item Illegal declaration of subroutine %s

(F) A subroutine was not declared correctly.  See L<perlsub>.

=item Illegal division by zero

(F) You tried to divide a number by 0.  Either something was wrong in
your logic, or you need to put a conditional in to guard against
meaningless input.

=item Illegal modulus zero

(F) You tried to divide a number by 0 to get the remainder.  Most
numbers don't take to this kindly.

=item Illegal number of bits in vec

(F) The number of bits in vec() (the third argument) must be a power of
two from 1 to 32 (or 64, if your platform supports that).

=item Illegal octal digit '%c'

(F) You used an 8 or 9 in an octal number.

=item Illegal operator following parameter in a subroutine signature

(F) A parameter in a subroutine signature, was followed by something
other than C<=> introducing a default, C<,> or C<)>.

    use feature 'signatures';
    sub foo ($=1) {}           # legal
    sub foo ($a = 1) {}        # legal
    sub foo ($a += 1) {}       # illegal
    sub foo ($a == 1) {}       # illegal

=item Illegal pattern in regex; marked by S<<-- HERE> in m/%s/

(F) You wrote something like

 (?+foo)

The C<"+"> is valid only when followed by digits, indicating a
capturing group.  See
L<C<(?I<PARNO>)>|perlre/(?PARNO) (?-PARNO) (?+PARNO) (?R) (?0)>.

=item Illegal suidscript

(F) The script run under suidperl was somehow illegal.

=item Illegal switch in PERL5OPT: -%c

(X) The PERL5OPT environment variable may only be used to set the
following switches: B<-[CDIMUdmtw]>.

=item Illegal user-defined property name

(F) You specified a Unicode-like property name in a regular expression
pattern (using C<\p{}> or C<\P{}>) that Perl knows isn't an official
Unicode property, and was likely meant to be a user-defined property
name, but it can't be one of those, as they must begin with either C<In>
or C<Is>.  Check the spelling.  See also
L</Can't find Unicode property definition "%s">.

=item Ill-formed CRTL environ value "%s"

(W internal) A warning peculiar to VMS.  Perl tried to read the CRTL's
internal environ array, and encountered an element without the C<=>
delimiter used to separate keys from values.  The element is ignored.

=item Ill-formed message in prime_env_iter: |%s|

(W internal) A warning peculiar to VMS.  Perl tried to read a logical
name or CLI symbol definition when preparing to iterate over %ENV, and
didn't see the expected delimiter between key and value, so the line was
ignored.

=item (in cleanup) %s

(W misc) This prefix usually indicates that a DESTROY() method raised
the indicated exception.  Since destructors are usually called by the
system at arbitrary points during execution, and often a vast number of
times, the warning is issued only once for any number of failures that
would otherwise result in the same message being repeated.

Failure of user callbacks dispatched using the C<G_KEEPERR> flag could
also result in this warning.  See L<perlcall/G_KEEPERR>.

=item Implicit use of @_ in %s with signatured subroutine is experimental

(S experimental::args_array_with_signatures) An expression that implicitly
involves the C<@_> arguments array was found in a subroutine that uses a
signature.  This is experimental because the interaction between the
arguments array and parameter handling via signatures is not guaranteed
to remain stable in any future version of Perl, and such code should be
avoided.

=item Incomplete expression within '(?[ ])' in regex; marked by S<<-- HERE>
in m/%s/

(F) There was a syntax error within the C<(?[ ])>.  This can happen if the
expression inside the construct was completely empty, or if there are
too many or few operands for the number of operators.  Perl is not smart
enough to give you a more precise indication as to what is wrong.

=item Inconsistent hierarchy during C3 merge of class '%s': merging failed on 
parent '%s'

(F) The method resolution order (MRO) of the given class is not
C3-consistent, and you have enabled the C3 MRO for this class.  See the C3
documentation in L<mro> for more information.

=item Indentation on line %d of here-doc doesn't match delimiter

(F) You have an indented here-document where one or more of its lines
have whitespace at the beginning that does not match the closing
delimiter.

For example, line 2 below is wrong because it does not have at least
2 spaces, but lines 1 and 3 are fine because they have at least 2:

    if ($something) {
      print <<~EOF;
        Line 1
       Line 2 not
          Line 3
        EOF
    }

Note that tabs and spaces are compared strictly, meaning 1 tab will
not match 8 spaces.

=item Infinite recursion in regex

(F) You used a pattern that references itself without consuming any input
text.  You should check the pattern to ensure that recursive patterns
either consume text or fail.

=item Infinite recursion in user-defined property

(F) A user-defined property (L<perlunicode/User-Defined Character
Properties>) can depend on the definitions of other user-defined
properties.  If the chain of dependencies leads back to this property,
infinite recursion would occur, were it not for the check that raised
this error.

Restructure your property definitions to avoid this.

=item Infinite recursion via empty pattern

(F) You tried to use the empty pattern inside of a regex code block,
for instance C</(?{ s!!! })/>, which resulted in re-executing
the same pattern, which is an infinite loop which is broken by
throwing an exception.

=item Initialization of state variables in list currently forbidden

(F) C<state> only permits initializing a single variable, specified
without parentheses.  So C<state $a = 42> and C<state @a = qw(a b c)> are
allowed, but not C<state ($a) = 42> or C<(state $a) = 42>.  To initialize
more than one C<state> variable, initialize them one at a time.

=item %%s[%s] in scalar context better written as $%s[%s]

(W syntax) In scalar context, you've used an array index/value slice
(indicated by %) to select a single element of an array.  Generally
it's better to ask for a scalar value (indicated by $).  The difference
is that C<$foo[&bar]> always behaves like a scalar, both in the value it
returns and when evaluating its argument, while C<%foo[&bar]> provides
a list context to its subscript, which can do weird things if you're
expecting only one subscript.  When called in list context, it also
returns the index (what C<&bar> returns) in addition to the value.

=item %%s{%s} in scalar context better written as $%s{%s}

(W syntax) In scalar context, you've used a hash key/value slice
(indicated by %) to select a single element of a hash.  Generally it's
better to ask for a scalar value (indicated by $).  The difference
is that C<$foo{&bar}> always behaves like a scalar, both in the value
it returns and when evaluating its argument, while C<@foo{&bar}> and
provides a list context to its subscript, which can do weird things
if you're expecting only one subscript.  When called in list context,
it also returns the key in addition to the value.

=item Insecure dependency in %s

(F) You tried to do something that the tainting mechanism didn't like.
The tainting mechanism is turned on when you're running setuid or
setgid, or when you specify B<-T> to turn it on explicitly.  The
tainting mechanism labels all data that's derived directly or indirectly
from the user, who is considered to be unworthy of your trust.  If any
such data is used in a "dangerous" operation, you get this error.  See
L<perlsec> for more information.

=item Insecure directory in %s

(F) You can't use system(), exec(), or a piped open in a setuid or
setgid script if C<$ENV{PATH}> contains a directory that is writable by
the world.  Also, the PATH must not contain any relative directory.
See L<perlsec>.

=item Insecure $ENV{%s} while running %s

(F) You can't use system(), exec(), or a piped open in a setuid or
setgid script if any of C<$ENV{PATH}>, C<$ENV{IFS}>, C<$ENV{CDPATH}>,
C<$ENV{ENV}>, C<$ENV{BASH_ENV}> or C<$ENV{TERM}> are derived from data
supplied (or potentially supplied) by the user.  The script must set
the path to a known value, using trustworthy data.  See L<perlsec>.

=item Insecure user-defined property %s

(F) Perl detected tainted data when trying to compile a regular
expression that contains a call to a user-defined character property
function, i.e. C<\p{IsFoo}> or C<\p{InFoo}>.
See L<perlunicode/User-Defined Character Properties> and L<perlsec>.

=item Integer overflow in format string for %s

(F) The indexes and widths specified in the format string of C<printf()>
or C<sprintf()> are too large.  The numbers must not overflow the size of
integers for your architecture.

=item Integer overflow in %s number

(S overflow) The hexadecimal, octal or binary number you have specified
either as a literal or as an argument to hex() or oct() is too big for
your architecture, and has been converted to a floating point number.
On a 32-bit architecture the largest hexadecimal, octal or binary number
representable without overflow is 0xFFFFFFFF, 037777777777, or
0b11111111111111111111111111111111 respectively.  Note that Perl
transparently promotes all numbers to a floating point representation
internally--subject to loss of precision errors in subsequent
operations.

=item Integer overflow in srand

(S overflow) The number you have passed to srand is too big to fit
in your architecture's integer representation.  The number has been
replaced with the largest integer supported (0xFFFFFFFF on 32-bit
architectures).  This means you may be getting less randomness than
you expect, because different random seeds above the maximum will
return the same sequence of random numbers.

=item Integer overflow in version

=item Integer overflow in version %d

(W overflow) Some portion of a version initialization is too large for
the size of integers for your architecture.  This is not a warning
because there is no rational reason for a version to try and use an
element larger than typically 2**32.  This is usually caused by trying
to use some odd mathematical operation as a version, like 100/9.

=item Internal disaster in regex; marked by S<<-- HERE> in m/%s/

(P) Something went badly wrong in the regular expression parser.
The S<<-- HERE> shows whereabouts in the regular expression the problem was
discovered.

=item Internal inconsistency in tracking vforks

(S) A warning peculiar to VMS.  Perl keeps track of the number of times
you've called C<fork> and C<exec>, to determine whether the current call
to C<exec> should affect the current script or a subprocess (see
L<perlvms/"exec LIST">).  Somehow, this count has become scrambled, so
Perl is making a guess and treating this C<exec> as a request to
terminate the Perl script and execute the specified command.

=item internal %<num>p might conflict with future printf extensions

(S internal) Perl's internal routine that handles C<printf> and C<sprintf>
formatting follows a slightly different set of rules when called from
C or XS code.  Specifically, formats consisting of digits followed
by "p" (e.g., "%7p") are reserved for future use.  If you see this
message, then an XS module tried to call that routine with one such
reserved format.

=item Internal urp in regex; marked by S<<-- HERE> in m/%s/

(P) Something went badly awry in the regular expression parser.  The
S<<-- HERE> shows whereabouts in the regular expression the problem was
discovered.

=item %s (...) interpreted as function

(W syntax) You've run afoul of the rule that says that any list operator
followed by parentheses turns into a function, with all the list
operators arguments found inside the parentheses.  See
L<perlop/Terms and List Operators (Leftward)>.

=item In '(?...)', the '(' and '?' must be adjacent in regex;
marked by S<<-- HERE> in m/%s/

(F) The two-character sequence C<"(?"> in this context in a regular
expression pattern should be an indivisible token, with nothing
intervening between the C<"("> and the C<"?">, but you separated them
with whitespace.

=item In '(*...)', the '(' and '*' must be adjacent in regex;
marked by S<<-- HERE> in m/%s/

(F) The two-character sequence C<"(*"> in this context in a regular
expression pattern should be an indivisible token, with nothing
intervening between the C<"("> and the C<"*">, but you separated them.
Fix the pattern and retry.

=item Invalid %s attribute: %s

(F) The indicated attribute for a subroutine or variable was not recognized
by Perl or by a user-supplied handler.  See L<attributes>.

=item Invalid %s attributes: %s

(F) The indicated attributes for a subroutine or variable were not
recognized by Perl or by a user-supplied handler.  See L<attributes>.

=item Invalid character in charnames alias definition; marked by
S<<-- HERE> in '%s

(F) You tried to create a custom alias for a character name, with
the C<:alias> option to C<use charnames> and the specified character in
the indicated name isn't valid.  See L<charnames/CUSTOM ALIASES>.

=item Invalid \0 character in %s for %s: %s\0%s

(W syscalls) Embedded \0 characters in pathnames or other system call
arguments produce a warning as of 5.20.  The parts after the \0 were
formerly ignored by system calls.

=item Invalid character in \N{...}; marked by S<<-- HERE> in \N{%s}

(F) Only certain characters are valid for character names.  The
indicated one isn't.  See L<charnames/CUSTOM ALIASES>.

=item Invalid conversion in %s: "%s"

(W printf) Perl does not understand the given format conversion.  See
L<perlfunc/sprintf>.

=item Invalid escape in the specified encoding in regex; marked by
S<<-- HERE> in m/%s/

(W regexp)(F) The numeric escape (for example C<\xHH>) of value < 256
didn't correspond to a single character through the conversion
from the encoding specified by the encoding pragma.
The escape was replaced with REPLACEMENT CHARACTER (U+FFFD)
instead, except within S<C<(?[   ])>>, where it is a fatal error.
The S<<-- HERE> shows whereabouts in the regular expression the
escape was discovered.

=item Invalid hexadecimal number in \N{U+...}

=item Invalid hexadecimal number in \N{U+...} in regex; marked by
S<<-- HERE> in m/%s/

(F) The character constant represented by C<...> is not a valid hexadecimal
number.  Either it is empty, or you tried to use a character other than
0 - 9 or A - F, a - f in a hexadecimal number.

=item Invalid module name %s with -%c option: contains single ':'

(F) The module argument to perl's B<-m> and B<-M> command-line options
cannot contain single colons in the module name, but only in the
arguments after "=".  In other words, B<-MFoo::Bar=:baz> is ok, but
B<-MFoo:Bar=baz> is not.

=item Invalid mro name: '%s'

(F) You tried to C<mro::set_mro("classname", "foo")> or C<use mro 'foo'>,
where C<foo> is not a valid method resolution order (MRO).  Currently,
the only valid ones supported are C<dfs> and C<c3>, unless you have loaded
a module that is a MRO plugin.  See L<mro> and L<perlmroapi>.

=item Invalid negative number (%s) in chr

(W utf8) You passed a negative number to C<chr>.  Negative numbers are
not valid character numbers, so it returns the Unicode replacement
character (U+FFFD).

=item Invalid number '%s' for -C option.

(F) You supplied a number to the -C option that either has extra leading
zeroes or overflows perl's unsigned integer representation.

=item invalid option -D%c, use -D'' to see choices

(S debugging) Perl was called with invalid debugger flags.  Call perl
with the B<-D> option with no flags to see the list of acceptable values.
See also L<perlrun/-Dletters>.

=item Invalid quantifier in {,} in regex; marked by S<<-- HERE> in m/%s/

(F) The pattern looks like a {min,max} quantifier, but the min or max
could not be parsed as a valid number - either it has leading zeroes,
or it represents too big a number to cope with.  The S<<-- HERE> shows
where in the regular expression the problem was discovered.  See L<perlre>.

=item Invalid [] range "%s" in regex; marked by S<<-- HERE> in m/%s/

(F) The range specified in a character class had a minimum character
greater than the maximum character.  One possibility is that you forgot the
C<{}> from your ending C<\x{}> - C<\x> without the curly braces can go only
up to C<ff>.  The S<<-- HERE> shows whereabouts in the regular expression the
problem was discovered.  See L<perlre>.

=item Invalid range "%s" in transliteration operator

(F) The range specified in the tr/// or y/// operator had a minimum
character greater than the maximum character.  See L<perlop>.

=item Invalid reference to group in regex; marked by S<<-- HERE> in m/%s/

(F) The capture group you specified can't possibly exist because the
number you used is not within the legal range of possible values for
this machine.

=item Invalid separator character %s in attribute list

(F) Something other than a colon or whitespace was seen between the
elements of an attribute list.  If the previous attribute had a
parenthesised parameter list, perhaps that list was terminated too soon.
See L<attributes>.

=item Invalid separator character %s in PerlIO layer specification %s

(W layer) When pushing layers onto the Perl I/O system, something other
than a colon or whitespace was seen between the elements of a layer list.
If the previous attribute had a parenthesised parameter list, perhaps that
list was terminated too soon.

=item Invalid strict version format (%s)

(F) A version number did not meet the "strict" criteria for versions.
A "strict" version number is a positive decimal number (integer or
decimal-fraction) without exponentiation or else a dotted-decimal
v-string with a leading 'v' character and at least three components.
The parenthesized text indicates which criteria were not met.
See the L<version> module for more details on allowed version formats.

=item Invalid type '%s' in %s

(F) The given character is not a valid pack or unpack type.
See L<perlfunc/pack>.

(W) The given character is not a valid pack or unpack type but used to be
silently ignored.

=item Invalid version format (%s)

(F) A version number did not meet the "lax" criteria for versions.
A "lax" version number is a positive decimal number (integer or
decimal-fraction) without exponentiation or else a dotted-decimal
v-string.  If the v-string has fewer than three components, it
must have a leading 'v' character.  Otherwise, the leading 'v' is
optional.  Both decimal and dotted-decimal versions may have a
trailing "alpha" component separated by an underscore character
after a fractional or dotted-decimal component.  The parenthesized
text indicates which criteria were not met.  See the L<version> module
for more details on allowed version formats.

=item Invalid version object

(F) The internal structure of the version object was invalid.
Perhaps the internals were modified directly in some way or
an arbitrary reference was blessed into the "version" class.

=item In '(*VERB...)', the '(' and '*' must be adjacent in regex;
marked by S<<-- HERE> in m/%s/

=item Inverting a character class which contains a multi-character
sequence is illegal in regex; marked by <-- HERE in m/%s/

(F) You wrote something like

 qr/\P{name=KATAKANA LETTER AINU P}/
 qr/[^\p{name=KATAKANA LETTER AINU P}]/

This name actually evaluates to a sequence of two Katakana characters,
not just a single one, and it is illegal to try to take the complement
of a sequence.  (Mathematically it would mean any sequence of characters
from 0 to infinity in length that weren't these two in a row, and that
is likely not of any real use.)

(F) The two-character sequence C<"(*"> in this context in a regular
expression pattern should be an indivisible token, with nothing
intervening between the C<"("> and the C<"*">, but you separated them.

=item ioctl is not implemented

(F) Your machine apparently doesn't implement ioctl(), which is pretty
strange for a machine that supports C.

=item ioctl() on unopened %s

(W unopened) You tried ioctl() on a filehandle that was never opened.
Check your control flow and number of arguments.

=item IO layers (like '%s') unavailable

(F) Your Perl has not been configured to have PerlIO, and therefore
you cannot use IO layers.  To have PerlIO, Perl must be configured
with 'useperlio'.

=item IO::Socket::atmark not implemented on this architecture

(F) Your machine doesn't implement the sockatmark() functionality,
neither as a system call nor an ioctl call (SIOCATMARK).

=item '%s' is an unknown bound type in regex; marked by S<<-- HERE> in m/%s/

(F) You used C<\b{...}> or C<\B{...}> and the C<...> is not known to
Perl.  The current valid ones are given in
L<perlrebackslash/\b{}, \b, \B{}, \B>.

=item %s() isn't allowed on :utf8 handles

(F) The sysread(), recv(), syswrite() and send() operators are
not allowed on handles that have the C<:utf8> layer, either explicitly, or
implicitly, eg., with the C<:encoding(UTF-16LE)> layer.

Previously sysread() and recv() currently use only the C<:utf8> flag for the stream,
ignoring the actual layers.  Since sysread() and recv() did no UTF-8
validation they can end up creating invalidly encoded scalars.

Similarly, syswrite() and send() used only the C<:utf8> flag, otherwise ignoring
any layers.  If the flag is set, both wrote the value UTF-8 encoded, even if
the layer is some different encoding, such as the example above.

Ideally, all of these operators would completely ignore the C<:utf8> state,
working only with bytes, but this would result in silently breaking existing
code.

=item "%s" is more clearly written simply as "%s" in regex; marked by S<<-- HERE> in m/%s/

(W regexp) (only under C<S<use re 'strict'>> or within C<(?[...])>)

You specified a character that has the given plainer way of writing it, and
which is also portable to platforms running with different character sets.

=item $* is no longer supported as of Perl 5.30

(F) The special variable C<$*>, deprecated in older perls, was removed in
5.10.0, is no longer supported and is a fatal error as of Perl 5.30.  In
previous versions of perl the use of C<$*> enabled or disabled multi-line
matching within a string.

Instead of using C<$*> you should use the C</m> (and maybe C</s>) regexp
modifiers.  You can enable C</m> for a lexical scope (even a whole file)
with C<use re '/m'>.  (In older versions: when C<$*> was set to a true value
then all regular expressions behaved as if they were written using C</m>.)

Use of this variable will be a fatal error in Perl 5.30.

=item $# is no longer supported as of Perl 5.30

(F) The special variable C<$#>, deprecated in older perls, was removed as of
5.10.0, is no longer supported and is a fatal error as of Perl 5.30.  You
should use the printf/sprintf functions instead.

=item '%s' is not a code reference

(W overload) The second (fourth, sixth, ...) argument of
overload::constant needs to be a code reference.  Either
an anonymous subroutine, or a reference to a subroutine.

=item '%s' is not an overloadable type

(W overload) You tried to overload a constant type the overload package is
unaware of.

=item '%s' is not recognised as a builtin function

(F) An attempt was made to C<use> the L<builtin> pragma module to create
a lexical alias for an unknown function name.

=item -i used with no filenames on the command line, reading from STDIN

(S inplace) The C<-i> option was passed on the command line, indicating
that the script is intended to edit files in place, but no files were
given.  This is usually a mistake, since editing STDIN in place doesn't
make sense, and can be confusing because it can make perl look like
it is hanging when it is really just trying to read from STDIN.  You
should either pass a filename to edit, or remove C<-i> from the command
line.  See L<perlrun|perlrun/-i[extension]> for more details.

=item Junk on end of regexp in regex m/%s/

(P) The regular expression parser is confused.

=item \K not permitted in lookahead/lookbehind in regex; marked by <-- HERE in m/%s/

(F) Your regular expression used C<\K> in a lookahead or lookbehind
assertion, which currently isn't permitted.

This may change in the future, see L<Support \K in
lookarounds|https://github.com/Perl/perl5/issues/18134>.

=item Label not found for "last %s"

(F) You named a loop to break out of, but you're not currently in a loop
of that name, not even if you count where you were called from.  See
L<perlfunc/last>.

=item Label not found for "next %s"

(F) You named a loop to continue, but you're not currently in a loop of
that name, not even if you count where you were called from.  See
L<perlfunc/last>.

=item Label not found for "redo %s"

(F) You named a loop to restart, but you're not currently in a loop of
that name, not even if you count where you were called from.  See
L<perlfunc/last>.

=item leaving effective %s failed

(F) While under the C<use filetest> pragma, switching the real and
effective uids or gids failed.

=item length/code after end of string in unpack

(F) While unpacking, the string buffer was already used up when an unpack
length/code combination tried to obtain more data.  This results in
an undefined value for the length.  See L<perlfunc/pack>.

=item length() used on %s (did you mean "scalar(%s)"?)

(W syntax) You used length() on either an array or a hash when you
probably wanted a count of the items.

Array size can be obtained by doing:

    scalar(@array);

The number of items in a hash can be obtained by doing:

    scalar(keys %hash);

=item Lexing code attempted to stuff non-Latin-1 character into Latin-1 input

(F) An extension is attempting to insert text into the current parse
(using L<lex_stuff_pvn|perlapi/lex_stuff_pvn> or similar), but tried to insert a character that
couldn't be part of the current input.  This is an inherent pitfall
of the stuffing mechanism, and one of the reasons to avoid it.  Where
it is necessary to stuff, stuffing only plain ASCII is recommended.

=item Lexing code internal error (%s)

(F) Lexing code supplied by an extension violated the lexer's API in a
detectable way.

=item listen() on closed socket %s

(W closed) You tried to do a listen on a closed socket.  Did you forget
to check the return value of your socket() call?  See
L<perlfunc/listen>.

=item List form of piped open not implemented

(F) On some platforms, notably Windows, the three-or-more-arguments
form of C<open> does not support pipes, such as C<open($pipe, '|-', @args)>.
Use the two-argument C<open($pipe, '|prog arg1 arg2...')> form instead.

=item Literal vertical space in [] is illegal except under /x in regex;
marked by S<<-- HERE> in m/%s/

(F) (only under C<S<use re 'strict'>> or within C<(?[...])>)

Likely you forgot the C</x> modifier or there was a typo in the pattern.
For example, did you really mean to match a form-feed?  If so, all the
ASCII vertical space control characters are representable by escape
sequences which won't present such a jarring appearance as your pattern
does when displayed.

  \r    carriage return
  \f    form feed
  \n    line feed
  \cK   vertical tab

=item %s: loadable library and perl binaries are mismatched (got %s handshake key %p, needed %p)

(P) A dynamic loading library C<.so> or C<.dll> was being loaded into the
process that was built against a different build of perl than the
said library was compiled against.  Reinstalling the XS module will
likely fix this error.

=item Locale '%s' contains (at least) the following characters which
have unexpected meanings: %s  The Perl program will use the expected
meanings

(W locale) You are using the named UTF-8 locale.  UTF-8 locales are
expected to have very particular behavior, which most do.  This message
arises when perl found some departures from the expectations, and is
notifying you that the expected behavior overrides these differences.
In some cases the differences are caused by the locale definition being
defective, but the most common causes of this warning are when there are
ambiguities and conflicts in following the Standard, and the locale has
chosen an approach that differs from Perl's.

One of these is because that, contrary to the claims, Unicode is not
completely locale insensitive.  Turkish and some related languages
have two types of C<"I"> characters.  One is dotted in both upper- and
lowercase, and the other is dotless in both cases.  Unicode allows a
locale to use either the Turkish rules, or the rules used in all other
instances, where there is only one type of C<"I">, which is dotless in
the uppercase, and dotted in the lower.  The perl core does not (yet)
handle the Turkish case, and this message warns you of that.  Instead,
the L<Unicode::Casing> module allows you to mostly implement the Turkish
casing rules.

The other common cause is for the characters

 $ + < = > ^ ` | ~

These are problematic.  The C standard says that these should be
considered punctuation in the C locale (and the POSIX standard defers to
the C standard), and Unicode is generally considered a superset of
the C locale.  But Unicode has added an extra category, "Symbol", and
classifies these particular characters as being symbols.  Most UTF-8
locales have them treated as punctuation, so that L<ispunct(3)> returns
non-zero for them.  But a few locales have it return 0.   Perl takes
the first approach, not using C<ispunct()> at all (see L<Note [5] in
perlrecharclass|perlrecharclass/[5]>), and this message is raised to notify you that you
are getting Perl's approach, not the locale's.

=item Locale '%s' is unsupported, and may crash the interpreter.

(S locale) The named locale is not supported by Perl, and using it leads
to undefined behavior, including potentially crashing the computer.

Currently the only locales that generate this severe warning are
non-UTF-8 ones which have characters that require more than one byte to
represent (common in older East Asian language locales).  See
L<perllocale>.

=item Locale '%s' may not work well.%s

(W locale) You are using the named locale, which is a non-UTF-8 one, and
which perl has determined is not fully compatible with what it can
handle.  The second C<%s> gives a reason.

By far the most common reason is that the locale has characters in it
that are represented by more than one byte.  The only such locales that
Perl can handle are the UTF-8 locales.  Most likely the specified locale
is a non-UTF-8 one for an East Asian language such as Chinese or
Japanese.  If the locale is a superset of ASCII, the ASCII portion of it
may work in Perl.

Some essentially obsolete locales that aren't supersets of ASCII, mainly
those in ISO 646 or other 7-bit locales, such as ASMO 449, can also have
problems, depending on what portions of the ASCII character set get
changed by the locale and are also used by the program.
The warning message lists the determinable conflicting characters.

Note that not all incompatibilities are found.

If this happens to you, there's not much you can do except switch to use a
different locale or use L<Encode> to translate from the locale into
UTF-8; if that's impracticable, you have been warned that some things
may break.

This message is output once each time a bad locale is switched into
within the scope of C<S<use locale>>, or on the first possibly-affected
operation if the C<S<use locale>> inherits a bad one.  It is not raised
for any operations from the L<POSIX> module.

=item localtime(%f) failed

(W overflow) You called C<localtime> with a number that it could not handle:
too large, too small, or NaN.  The returned value is C<undef>.

=item localtime(%f) too large

(W overflow) You called C<localtime> with a number that was larger
than it can reliably handle and C<localtime> probably returned the
wrong date.  This warning is also triggered with NaN (the special
not-a-number value).

=item localtime(%f) too small

(W overflow) You called C<localtime> with a number that was smaller
than it can reliably handle and C<localtime> probably returned the
wrong date.

=item Lookbehind longer than %d not implemented in regex m/%s/

(F) There is currently a limit on the length of string which lookbehind can
handle.  This restriction may be eased in a future release. 

=item Lost precision when %s %f by 1

(W imprecision) You attempted to increment or decrement a value by one,
but the result is too large for the underlying floating point
representation to store accurately. Hence, the target of C<++> or C<-->
is increased or decreased by quite different value than one, such as
zero (I<i.e.> the target is unchanged) or two, due to rounding.
Perl issues this
warning because it has already switched from integers to floating point
when values are too large for integers, and now even floating point is
insufficient.  You may wish to switch to using L<Math::BigInt> explicitly.

=item lstat() on filehandle%s

(W io) You tried to do an lstat on a filehandle.  What did you mean
by that?  lstat() makes sense only on filenames.  (Perl did a fstat()
instead on the filehandle.)

=item lvalue attribute %s already-defined subroutine

(W misc) Although L<attributes.pm|attributes> allows this, turning the lvalue
attribute on or off on a Perl subroutine that is already defined
does not always work properly.  It may or may not do what you
want, depending on what code is inside the subroutine, with exact
details subject to change between Perl versions.  Only do this
if you really know what you are doing.

=item lvalue attribute ignored after the subroutine has been defined

(W misc) Using the C<:lvalue> declarative syntax to make a Perl
subroutine an lvalue subroutine after it has been defined is
not permitted.  To make the subroutine an lvalue subroutine,
add the lvalue attribute to the definition, or put the C<sub
foo :lvalue;> declaration before the definition.

See also L<attributes.pm|attributes>.

=item Magical list constants are not supported

(F) You assigned a magical array to a stash element, and then tried
to use the subroutine from the same slot.  You are asking Perl to do
something it cannot do, details subject to change between Perl versions.

=item Malformed integer in [] in pack

(F) Between the brackets enclosing a numeric repeat count only digits
are permitted.  See L<perlfunc/pack>.

=item Malformed integer in [] in unpack

(F) Between the brackets enclosing a numeric repeat count only digits
are permitted.  See L<perlfunc/pack>.

=item Malformed PERLLIB_PREFIX

(F) An error peculiar to OS/2.  PERLLIB_PREFIX should be of the form

    prefix1;prefix2

or
    prefix1 prefix2

with nonempty prefix1 and prefix2.  If C<prefix1> is indeed a prefix of
a builtin library search path, prefix2 is substituted.  The error may
appear if components are not found, or are too long.  See
"PERLLIB_PREFIX" in L<perlos2>.

=item Malformed prototype for %s: %s

(F) You tried to use a function with a malformed prototype.  The
syntax of function prototypes is given a brief compile-time check for
obvious errors like invalid characters.  A more rigorous check is run
when the function is called.
Perhaps the function's author was trying to write a subroutine signature
but didn't enable that feature first (C<use feature 'signatures'>),
so the signature was instead interpreted as a bad prototype.

=item Malformed UTF-8 character%s

(S utf8)(F) Perl detected a string that should be UTF-8, but didn't
comply with UTF-8 encoding rules, or represents a code point whose
ordinal integer value doesn't fit into the word size of the current
platform (overflows).  Details as to the exact malformation are given in
the variable, C<%s>, part of the message.

One possible cause is that you set the UTF8 flag yourself for data that
you thought to be in UTF-8 but it wasn't (it was for example legacy 8-bit
data).  To guard against this, you can use C<Encode::decode('UTF-8', ...)>.

If you use the C<:encoding(UTF-8)> PerlIO layer for input, invalid byte
sequences are handled gracefully, but if you use C<:utf8>, the flag is set
without validating the data, possibly resulting in this error message.

See also L<Encode/"Handling Malformed Data">.

=item Malformed UTF-8 returned by \N{%s} immediately after '%s'

(F) The charnames handler returned malformed UTF-8.

=item Malformed UTF-8 string in "%s"

(F) This message indicates a bug either in the Perl core or in XS
code. Such code was trying to find out if a character, allegedly
stored internally encoded as UTF-8, was of a given type, such as
being punctuation or a digit.  But the character was not encoded
in legal UTF-8.  The C<%s> is replaced by a string that can be used
by knowledgeable people to determine what the type being checked
against was.

Passing malformed strings was deprecated in Perl 5.18, and
became fatal in Perl 5.26.

=item Malformed UTF-8 string in '%c' format in unpack

(F) You tried to unpack something that didn't comply with UTF-8 encoding
rules and perl was unable to guess how to make more progress.

=item Malformed UTF-8 string in pack

(F) You tried to pack something that didn't comply with UTF-8 encoding
rules and perl was unable to guess how to make more progress.

=item Malformed UTF-8 string in unpack

(F) You tried to unpack something that didn't comply with UTF-8 encoding
rules and perl was unable to guess how to make more progress.

=item Malformed UTF-16 surrogate

(F) Perl thought it was reading UTF-16 encoded character data but while
doing it Perl met a malformed Unicode surrogate.

=item Mandatory parameter follows optional parameter

(F) In a subroutine signature, you wrote something like "$a = undef,
$b", making an earlier parameter optional and a later one mandatory.
Parameters are filled from left to right, so it's impossible for the
caller to omit an earlier one and pass a later one.  If you want to act
as if the parameters are filled from right to left, declare the rightmost
optional and then shuffle the parameters around in the subroutine's body.

=item Matched non-Unicode code point 0x%X against Unicode property; may
not be portable

(S non_unicode) Perl allows strings to contain a superset of
Unicode code points; each code point may be as large as what is storable
in a signed integer on your system, but these may not be accepted by
other languages/systems.  This message occurs when you matched a string
containing such a code point against a regular expression pattern, and
the code point was matched against a Unicode property, C<\p{...}> or
C<\P{...}>.  Unicode properties are only defined on Unicode code points,
so the result of this match is undefined by Unicode, but Perl (starting
in v5.20) treats non-Unicode code points as if they were typical
unassigned Unicode ones, and matched this one accordingly.  Whether a
given property matches these code points or not is specified in
L<perluniprops/Properties accessible through \p{} and \P{}>.

This message is suppressed (unless it has been made fatal) if it is
immaterial to the results of the match if the code point is Unicode or
not.  For example, the property C<\p{ASCII_Hex_Digit}> only can match
the 22 characters C<[0-9A-Fa-f]>, so obviously all other code points,
Unicode or not, won't match it.  (And C<\P{ASCII_Hex_Digit}> will match
every code point except these 22.)

Getting this message indicates that the outcome of the match arguably
should have been the opposite of what actually happened.  If you think
that is the case, you may wish to make the C<non_unicode> warnings
category fatal; if you agree with Perl's decision, you may wish to turn
off this category.

See L<perlunicode/Beyond Unicode code points> for more information.

=item %s matches null string many times in regex; marked by S<<-- HERE> in
m/%s/

(W regexp) The pattern you've specified would be an infinite loop if the
regular expression engine didn't specifically check for that.  The S<<-- HERE>
shows whereabouts in the regular expression the problem was discovered.
See L<perlre>.

=item Maximal count of pending signals (%u) exceeded

(F) Perl aborted due to too high a number of signals pending.  This
usually indicates that your operating system tried to deliver signals
too fast (with a very high priority), starving the perl process from
resources it would need to reach a point where it can process signals
safely.  (See L<perlipc/"Deferred Signals (Safe Signals)">.)

=item "%s" may clash with future reserved word

(W) This warning may be due to running a perl5 script through a perl4
interpreter, especially if the word that is being warned about is
"use" or "my".

=item '%' may not be used in pack

(F) You can't pack a string by supplying a checksum, because the
checksumming process loses information, and you can't go the other way.
See L<perlfunc/unpack>.

=item Method for operation %s not found in package %s during blessing

(F) An attempt was made to specify an entry in an overloading table that
doesn't resolve to a valid subroutine.  See L<overload>.

=item method is experimental

(S experimental::class) This warning is emitted if you use the C<method>
keyword of C<use feature 'class'>.  This keyword is currently
experimental and its behaviour may change in future releases of Perl.

=item Method %s not permitted

See L</500 Server error>.

=item Method %s redefined

(W redefine) You redefined a method.  To suppress this warning, say

    {
	no warnings 'redefine';
	*name = method { ... };
    }

=item Might be a runaway multi-line %s string starting on line %d

(S) An advisory indicating that the previous error may have been caused
by a missing delimiter on a string or pattern, because it eventually
ended earlier on the current line.

=item Mismatched brackets in template

(F) A pack template could not be parsed because pairs of C<[...]> or
C<(...)> could not be matched up. See L<perlfunc/pack>.

=item Misplaced _ in number

(W syntax) An underscore (underbar) in a numeric constant did not
separate two digits.

=item Missing argument for %n in %s

(F) A C<%n> was used in a format string with no corresponding argument for
perl to write the current string length to.

=item Missing argument in %s

(W missing) You called a function with fewer arguments than other
arguments you supplied indicated would be needed.

Currently only emitted when a printf-type format required more
arguments than were supplied, but might be used in the future for
other cases where we can statically determine that arguments to
functions are missing, e.g. for the L<perlfunc/pack> function.

=item Missing argument to -%c

(F) The argument to the indicated command line switch must follow
immediately after the switch, without intervening spaces.

=item Missing braces on \N{}

=item Missing braces on \N{} in regex; marked by S<<-- HERE> in m/%s/

(F) Wrong syntax of character name literal C<\N{charname}> within
double-quotish context.  This can also happen when there is a space
(or comment) between the C<\N> and the C<{> in a regex with the C</x> modifier.
This modifier does not change the requirement that the brace immediately
follow the C<\N>.

=item Missing braces on \o{}

(F) A C<\o> must be followed immediately by a C<{> in double-quotish context.

=item Missing comma after first argument to %s function

(F) While certain functions allow you to specify a filehandle or an
"indirect object" before the argument list, this ain't one of them.

=item Missing command in piped open

(W pipe) You used the C<open(FH, "| command")> or
C<open(FH, "command |")> construction, but the command was missing or
blank.

=item Missing control char name in \c

(F) A double-quoted string ended with "\c", without the required control
character name.

=item Missing ']' in prototype for %s : %s

(W illegalproto) A grouping was started with C<[> but never closed with C<]>.

=item Missing name in "%s sub"

(F) The syntax for lexically scoped subroutines requires that
they have a name with which they can be found.

=item Missing $ on loop variable

(F) Apparently you've been programming in B<csh> too much.  Variables
are always mentioned with the $ in Perl, unlike in the shells, where it
can vary from one line to the next.

=item (Missing operator before %s?)

(S syntax) This is an educated guess made in conjunction with the message
"%s found where operator expected".  Often the missing operator is a comma.

=item Missing or undefined argument to %s

(F) You tried to call C<require> or C<do> with no argument or with an
undefined value as an argument.  Require expects either a package name or
a file-specification as an argument; do expects a filename.  See
L<perlfunc/require EXPR> and L<perlfunc/do EXPR>.

=item Missing or undefined argument to %s via %{^HOOK}{require__before}

(F) A C<%{^HOOK}{require__before}> hook rewrote the name of the file being
compiled with C<require> or C<do> with an empty string an undefined value
which is forbidden.  See L<perlvar/%{^HOOK}> and L<perlfunc/require EXPR>.

=item Missing right brace on \%c{} in regex; marked by S<<-- HERE> in m/%s/

(F) Missing right brace in C<\x{...}>, C<\p{...}>, C<\P{...}>, or C<\N{...}>.

=item Missing right brace on \N{}

=item Missing right brace on \N{} or unescaped left brace after \N

(F) C<\N> has two meanings.

The traditional one has it followed by a name enclosed in braces,
meaning the character (or sequence of characters) given by that
name.  Thus C<\N{ASTERISK}> is another way of writing C<*>, valid in both
double-quoted strings and regular expression patterns.  In patterns,
it doesn't have the meaning an unescaped C<*> does.

Starting in Perl 5.12.0, C<\N> also can have an additional meaning (only)
in patterns, namely to match a non-newline character.  (This is short
for C<[^\n]>, and like C<.> but is not affected by the C</s> regex modifier.)

This can lead to some ambiguities.  When C<\N> is not followed immediately
by a left brace, Perl assumes the C<[^\n]> meaning.  Also, if the braces
form a valid quantifier such as C<\N{3}> or C<\N{5,}>, Perl assumes that this
means to match the given quantity of non-newlines (in these examples,
3; and 5 or more, respectively).  In all other case, where there is a
C<\N{> and a matching C<}>, Perl assumes that a character name is desired.

However, if there is no matching C<}>, Perl doesn't know if it was
mistakenly omitted, or if C<[^\n]{> was desired, and raises this error.
If you meant the former, add the right brace; if you meant the latter,
escape the brace with a backslash, like so: C<\N\{>

=item Missing right curly or square bracket

(F) The lexer counted more opening curly or square brackets than closing
ones.  As a general rule, you'll find it's missing near the place you
were last editing.

=item (Missing semicolon on previous line?)

(S syntax) This is an educated guess made in conjunction with the message
"%s found where operator expected".  Don't automatically put a semicolon on
the previous line just because you saw this message.

=item Modification of a read-only value attempted

(F) You tried, directly or indirectly, to change the value of a
constant.  You didn't, of course, try "2 = 1", because the compiler
catches that.  But an easy way to do the same thing is:

    sub mod { $_[0] = 1 }
    mod(2);

Another way is to assign to a substr() that's off the end of the string.

Yet another way is to assign to a C<foreach> loop I<VAR> when I<VAR>
is aliased to a constant in the look I<LIST>:

    $x = 1;
    foreach my $n ($x, 2) {
        $n *= 2; # modifies the $x, but fails on attempt to
    }            # modify the 2

L<PerlIO::scalar> will also produce this message as a warning if you
attempt to open a read-only scalar for writing.

=item Modification of non-creatable array value attempted, %s

(F) You tried to make an array value spring into existence, and the
subscript was probably negative, even counting from end of the array
backwards.

=item Modification of non-creatable hash value attempted, %s

(P) You tried to make a hash value spring into existence, and it
couldn't be created for some peculiar reason.

=item Module name must be constant

(F) Only a bare module name is allowed as the first argument to a "use".

=item Module name required with -%c option

(F) The C<-M> or C<-m> options say that Perl should load some module, but
you omitted the name of the module.  Consult
L<perlrun|perlrun/-m[-]module> for full details about C<-M> and C<-m>.

=item More than one argument to '%s' open

(F) The C<open> function has been asked to open multiple files.  This
can happen if you are trying to open a pipe to a command that takes a
list of arguments, but have forgotten to specify a piped open mode.
See L<perlfunc/open> for details.

=item mprotect for COW string %p %u failed with %d

(S) You compiled perl with B<-D>PERL_DEBUG_READONLY_COW (see
L<perlguts/"Copy on Write">), but a shared string buffer
could not be made read-only.

=item mprotect for %p %u failed with %d

(S) You compiled perl with B<-D>PERL_DEBUG_READONLY_OPS (see L<perlhacktips>),
but an op tree could not be made read-only.

=item mprotect RW for COW string %p %u failed with %d

(S) You compiled perl with B<-D>PERL_DEBUG_READONLY_COW (see
L<perlguts/"Copy on Write">), but a read-only shared string
buffer could not be made mutable.

=item mprotect RW for %p %u failed with %d

(S) You compiled perl with B<-D>PERL_DEBUG_READONLY_OPS (see
L<perlhacktips>), but a read-only op tree could not be made
mutable before freeing the ops.

=item msg%s not implemented

(F) You don't have System V message IPC on your system.

=item Multidimensional hash lookup is disabled

(F) You supplied a list of subscripts to a hash lookup under
C<< no feature "multidimensional"; >>, eg:

  $z = $foo{$x, $y};

which by default acts like:

  $z = $foo{join($;, $x, $y)};

=item Multidimensional syntax %s not supported

(W syntax) Multidimensional arrays aren't written like C<$foo[1,2,3]>.
They're written like C<$foo[1][2][3]>, as in C.

=item Multiple slurpy parameters not allowed

(F) In subroutine signatures, a slurpy parameter (C<@> or C<%>) must be
the last parameter, and there must not be more than one of them; for
example:

    sub foo ($a, @b)    {} # legal
    sub foo ($a, @b, %) {} # invalid

=item '/' must follow a numeric type in unpack

(F) You had an unpack template that contained a '/', but this did not
follow some unpack specification producing a numeric value.
See L<perlfunc/pack>.

=item %s must not be a named sequence in transliteration operator

(F) Transliteration (C<tr///> and C<y///>) transliterates individual
characters.  But a named sequence by definition is more than an
individual character, and hence doing this operation on it doesn't make
sense.

=item "my sub" not yet implemented

(F) Lexically scoped subroutines are not yet implemented.  Don't try
that yet.

=item "my" subroutine %s can't be in a package

(F) Lexically scoped subroutines aren't in a package, so it doesn't make
sense to try to declare one with a package qualifier on the front.

=item "my %s" used in sort comparison

(W syntax) The package variables $a and $b are used for sort comparisons.
You used $a or $b in as an operand to the C<< <=> >> or C<cmp> operator inside a
sort comparison block, and the variable had earlier been declared as a
lexical variable.  Either qualify the sort variable with the package
name, or rename the lexical variable.

=item "my" variable %s can't be in a package

(F) Lexically scoped variables aren't in a package, so it doesn't make
sense to try to declare one with a package qualifier on the front.  Use
local() if you want to localize a package variable.

=item Name "%s::%s" used only once: possible typo

(W once) Typographical errors often show up as unique variable
names.  If you had a good reason for having a unique name, then
just mention it again somehow to suppress the message.  The C<our>
declaration is also provided for this purpose.

NOTE: This warning detects package symbols that have been used
only once.  This means lexical variables will never trigger this
warning.  It also means that all of the package variables $c, @c,
%c, as well as *c, &c, sub c{}, c(), and c (the filehandle or
format) are considered the same; if a program uses $c only once
but also uses any of the others it will not trigger this warning.
Symbols beginning with an underscore and symbols using special
identifiers (q.v. L<perldata>) are exempt from this warning.

=item Need exactly 3 octal digits in regex; marked by S<<-- HERE> in m/%s/

(F) Within S<C<(?[   ])>>, all constants interpreted as octal need to be
exactly 3 digits long.  This helps catch some ambiguities.  If your
constant is too short, add leading zeros, like

 (?[ [ \078 ] ])     # Syntax error!
 (?[ [ \0078 ] ])    # Works
 (?[ [ \007 8 ] ])   # Clearer

The maximum number this construct can express is C<\777>.  If you
need a larger one, you need to use L<\o{}|perlrebackslash/Octal escapes> instead.  If you meant
two separate things, you need to separate them:

 (?[ [ \7776 ] ])        # Syntax error!
 (?[ [ \o{7776} ] ])     # One meaning
 (?[ [ \777 6 ] ])       # Another meaning
 (?[ [ \777 \006 ] ])    # Still another

=item Negative '/' count in unpack

(F) The length count obtained from a length/code unpack operation was
negative.  See L<perlfunc/pack>.

=item Negative length

(F) You tried to do a read/write/send/recv operation with a buffer
length that is less than 0.  This is difficult to imagine.

=item Negative offset to vec in lvalue context

(F) When C<vec> is called in an lvalue context, the second argument must be
greater than or equal to zero.

=item Negative repeat count does nothing

(W numeric) You tried to execute the
L<C<x>|perlop/Multiplicative Operators> repetition operator fewer than 0
times, which doesn't make sense.

=item Nested quantifiers in regex; marked by S<<-- HERE> in m/%s/

(F) You can't quantify a quantifier without intervening parentheses.
So things like ** or +* or ?* are illegal.  The S<<-- HERE> shows
whereabouts in the regular expression the problem was discovered.

Note that the minimal matching quantifiers, C<*?>, C<+?>, and
C<??> appear to be nested quantifiers, but aren't.  See L<perlre>.

=item %s never introduced

(S internal) The symbol in question was declared but somehow went out of
scope before it could possibly have been used.

=item next::method/next::can/maybe::next::method cannot find enclosing method

(F) C<next::method> needs to be called within the context of a
real method in a real package, and it could not find such a context.
See L<mro>.

=item \N in a character class must be a named character: \N{...} in regex; 
marked by S<<-- HERE> in m/%s/

(F) The new (as of Perl 5.12) meaning of C<\N> as C<[^\n]> is not valid in a
bracketed character class, for the same reason that C<.> in a character
class loses its specialness: it matches almost everything, which is
probably not what you want.

=item \N{} here is restricted to one character in regex; marked by <-- HERE in m/%s/

(F) Named Unicode character escapes (C<\N{...}>) may return a
multi-character sequence.  Even though a character class is
supposed to match just one character of input, perl will match the
whole thing correctly, except under certain conditions.  These currently
are

=over 4

=item When the class is inverted (C<[^...]>)

The mathematically logical behavior for what matches when inverting
is very different from what people expect, so we have decided to
forbid it.

=item The escape is the beginning or final end point of a range

Similarly unclear is what should be generated when the
C<\N{...}> is used as one of the end points of the range, such as in

 [\x{41}-\N{ARABIC SEQUENCE YEH WITH HAMZA ABOVE WITH AE}]

What is meant here is unclear, as the C<\N{...}> escape is a sequence
of code points, so this is made an error.

=item In a regex set

The syntax S<C<(?[   ])>> in a regular expression yields a list of
single code points, none can be a sequence.

=back

=item No %s allowed while running setuid

(F) Certain operations are deemed to be too insecure for a setuid or
setgid script to even be allowed to attempt.  Generally speaking there
will be another way to do what you want that is, if not secure, at least
securable.  See L<perlsec>.

=item No code specified for -%c

(F) Perl's B<-e> and B<-E> command-line options require an argument.  If
you want to run an empty program, pass the empty string as a separate
argument or run a program consisting of a single 0 or 1:

    perl -e ""
    perl -e0
    perl -e1

=item No comma allowed after %s

(F) A list operator that has a filehandle or "indirect object" is
not allowed to have a comma between that and the following arguments.
Otherwise it'd be just another one of the arguments.

One possible cause for this is that you expected to have imported
a constant to your name space with B<use> or B<import> while no such
importing took place, it may for example be that your operating
system does not support that particular constant.  Hopefully you did
use an explicit import list for the constants you expect to see;
please see L<perlfunc/use> and L<perlfunc/import>.  While an
explicit import list would probably have caught this error earlier
it naturally does not remedy the fact that your operating system
still does not support that constant.  Maybe you have a typo in
the constants of the symbol import list of B<use> or B<import> or in the
constant name at the line where this error was triggered?

=item No command into which to pipe on command line

(F) An error peculiar to VMS.  Perl handles its own command line
redirection, and found a '|' at the end of the command line, so it
doesn't know where you want to pipe the output from this command.

=item No DB::DB routine defined

(F) The currently executing code was compiled with the B<-d> switch, but
for some reason the current debugger (e.g. F<perl5db.pl> or a C<Devel::>
module) didn't define a routine to be called at the beginning of each
statement.

=item No dbm on this machine

(P) This is counted as an internal error, because every machine should
supply dbm nowadays, because Perl comes with SDBM.  See L<SDBM_File>.

=item No DB::sub routine defined

(F) The currently executing code was compiled with the B<-d> switch, but
for some reason the current debugger (e.g. F<perl5db.pl> or a C<Devel::>
module) didn't define a C<DB::sub> routine to be called at the beginning
of each ordinary subroutine call.

=item No digits found for %s literal

(F) No hexadecimal digits were found following C<0x> or no binary digits
were found following C<0b>.

=item No directory specified for -I

(F) The B<-I> command-line switch requires a directory name as part of the
I<same> argument.  Use B<-Ilib>, for instance.  B<-I lib> won't work.

=item No error file after 2> or 2>> on command line

(F) An error peculiar to VMS.  Perl handles its own command line
redirection, and found a '2>' or a '2>>' on the command line, but can't
find the name of the file to which to write data destined for stderr.

=item No group ending character '%c' found in template

(F) A pack or unpack template has an opening '(' or '[' without its
matching counterpart.  See L<perlfunc/pack>.

=item No input file after < on command line

(F) An error peculiar to VMS.  Perl handles its own command line
redirection, and found a '<' on the command line, but can't find the
name of the file from which to read data for stdin.

=item No next::method '%s' found for %s

(F) C<next::method> found no further instances of this method name
in the remaining packages of the MRO of this class.  If you don't want
it throwing an exception, use C<maybe::next::method>
or C<next::can>.  See L<mro>.

=item Non-finite repeat count does nothing

(W numeric) You tried to execute the
L<C<x>|perlop/Multiplicative Operators> repetition operator C<Inf> (or
C<-Inf>) or C<NaN> times, which doesn't make sense.

=item Non-hex character in regex; marked by S<<-- HERE> in m/%s/

(F) In a regular expression, there was a non-hexadecimal character where
a hex one was expected, like

 (?[ [ \xDG ] ])
 (?[ [ \x{DEKA} ] ])

=item Non-hex character '%c' terminates \x early.  Resolved as "%s"

(W digit) In parsing a hexadecimal numeric constant, a character was
unexpectedly encountered that isn't hexadecimal.  The resulting value
is as indicated.

Note that, within braces, every character starting with the first
non-hexadecimal up to the ending brace is ignored.

=item Non-octal character in regex; marked by S<<-- HERE> in m/%s/

(F) In a regular expression, there was a non-octal character where
an octal one was expected, like

 (?[ [ \o{1278} ] ])

=item Non-octal character '%c' terminates \o early.  Resolved as "%s"

(W digit) In parsing an octal numeric constant, a character was
unexpectedly encountered that isn't octal.  The resulting value
is as indicated.

When not using C<\o{...}>, you wrote something like C<\08>, or C<\179>
in a double-quotish string.  The resolution is as indicated, with all
but the last digit treated as a single character, specified in octal.
The last digit is the next character in the string.  To tell Perl that
this is indeed what you want, you can use the C<\o{ }> syntax, or use
exactly three digits to specify the octal for the character.

Note that, within braces, every character starting with the first
non-octal up to the ending brace is ignored.

=item "no" not allowed in expression

(F) The "no" keyword is recognized and executed at compile time, and
returns no useful value.  See L<perlmod>.

=item Non-string passed as bitmask

(W misc) A number has been passed as a bitmask argument to select().
Use the vec() function to construct the file descriptor bitmasks for
select.  See L<perlfunc/select>.

=item No output file after > on command line

(F) An error peculiar to VMS.  Perl handles its own command line
redirection, and found a lone '>' at the end of the command line, so it
doesn't know where you wanted to redirect stdout.

=item No output file after > or >> on command line

(F) An error peculiar to VMS.  Perl handles its own command line
redirection, and found a '>' or a '>>' on the command line, but can't
find the name of the file to which to write data destined for stdout.

=item No package name allowed for subroutine %s in "our"

=item No package name allowed for variable %s in "our"

(F) Fully qualified subroutine and variable names are not allowed in "our"
declarations, because that doesn't make much sense under existing rules.
Such syntax is reserved for future extensions.

=item No Perl script found in input

(F) You called C<perl -x>, but no line was found in the file beginning
with #! and containing the word "perl".

=item No setregid available

(F) Configure didn't find anything resembling the setregid() call for
your system.

=item No setreuid available

(F) Configure didn't find anything resembling the setreuid() call for
your system.

=item No such class %s

(F) You provided a class qualifier in a "my", "our" or "state"
declaration, but this class doesn't exist at this point in your program.

=item No such class field "%s" in variable %s of type %s

(F) You tried to access a key from a hash through the indicated typed
variable but that key is not allowed by the package of the same type.
The indicated package has restricted the set of allowed keys using the
L<fields> pragma.

=item No such hook: %s

(F) You specified a signal hook that was not recognized by Perl.
Currently, Perl accepts C<__DIE__> and C<__WARN__> as valid signal hooks.

=item No such pipe open

(P) An error peculiar to VMS.  The internal routine my_pclose() tried to
close a pipe which hadn't been opened.  This should have been caught
earlier as an attempt to close an unopened filehandle.

=item No such signal: SIG%s

(W signal) You specified a signal name as a subscript to %SIG that was
not recognized.  Say C<kill -l> in your shell to see the valid signal
names on your system.

=item No Unicode property value wildcard matches:

(W regexp) You specified a wildcard for a Unicode property value, but
there is no property value in the current Unicode release that matches
it.  Check your spelling.

=item Not a CODE reference

(F) Perl was trying to evaluate a reference to a code value (that is, a
subroutine), but found a reference to something else instead.  You can
use the ref() function to find out what kind of ref it really was.  See
also L<perlref>.

=item Not a GLOB reference

(F) Perl was trying to evaluate a reference to a "typeglob" (that is, a
symbol table entry that looks like C<*foo>), but found a reference to
something else instead.  You can use the ref() function to find out what
kind of ref it really was.  See L<perlref>.

=item Not a HASH reference

(F) Perl was trying to evaluate a reference to a hash value, but found a
reference to something else instead.  You can use the ref() function to
find out what kind of ref it really was.  See L<perlref>.

=item '#' not allowed immediately following a sigil in a subroutine signature

(F) In a subroutine signature definition, a comment following a sigil
(C<$>, C<@> or C<%>), needs to be separated by whitespace or a comma etc., in
particular to avoid confusion with the C<$#> variable.  For example:

    # bad
    sub f ($# ignore first arg
           , $b) {}
    # good
    sub f ($, # ignore first arg
           $b) {}

=item Not an ARRAY reference

(F) Perl was trying to evaluate a reference to an array value, but found
a reference to something else instead.  You can use the ref() function
to find out what kind of ref it really was.  See L<perlref>.

=item Not a SCALAR reference

(F) Perl was trying to evaluate a reference to a scalar value, but found
a reference to something else instead.  You can use the ref() function
to find out what kind of ref it really was.  See L<perlref>.

=item Not a subroutine reference

(F) Perl was trying to evaluate a reference to a code value (that is, a
subroutine), but found a reference to something else instead.  You can
use the ref() function to find out what kind of ref it really was.  See
also L<perlref>.

=item Not a subroutine reference in overload table

(F) An attempt was made to specify an entry in an overloading table that
doesn't somehow point to a valid subroutine.  See L<overload>.

=item Not enough arguments for %s

(F) The function requires more arguments than you specified.

=item Not enough format arguments

(W syntax) A format specified more picture fields than the next line
supplied.  See L<perlform>.

=item %s: not found

(A) You've accidentally run your script through the Bourne shell instead
of Perl.  Check the #! line, or manually feed your script into Perl
yourself.

=item no UTC offset information; assuming local time is UTC

(S) A warning peculiar to VMS.  Perl was unable to find the local
timezone offset, so it's assuming that local system time is equivalent
to UTC.  If it's not, define the logical name
F<SYS$TIMEZONE_DIFFERENTIAL> to translate to the number of seconds which
need to be added to UTC to get local time.

=item NULL OP IN RUN

(S debugging) Some internal routine called run() with a null opcode
pointer.

=item Null picture in formline

(F) The first argument to formline must be a valid format picture
specification.  It was found to be empty, which probably means you
supplied it an uninitialized value.  See L<perlform>.

=item NULL regexp parameter

(P) The internal pattern matching routines are out of their gourd.

=item Number too long

(F) Perl limits the representation of decimal numbers in programs to
about 250 characters.  You've exceeded that length.  Future
versions of Perl are likely to eliminate this arbitrary limitation.  In
the meantime, try using scientific notation (e.g. "1e6" instead of
"1_000_000").

=item Number with no digits

(F) Perl was looking for a number but found nothing that looked like
a number.  This happens, for example with C<\o{}>, with no number between
the braces.

=item Numeric format result too large

(F) The length of the result of a numeric format supplied to sprintf()
or printf() would have been too large for the underlying C function to
report.  This limit is typically 2GB.

=item Numeric variables with more than one digit may not start with '0'

(F) The only numeric variable which is allowed to start with a 0 is C<$0>,
and you mentioned a variable that starts with 0 that has more than one
digit. You probably want to remove the leading 0, or if the intent was
to express a variable name in octal you should convert to decimal.

=item Octal number > 037777777777 non-portable

(W portable) The octal number you specified is larger than 2**32-1
(4294967295) and therefore non-portable between systems.  See
L<perlport> for more on portability concerns.

=item Odd name/value argument for subroutine '%s'

(F) A subroutine using a slurpy hash parameter in its signature
received an odd number of arguments to populate the hash.  It requires
the arguments to be paired, with the same number of keys as values.
The caller of the subroutine is presumably at fault.

The message attempts to include the name of the called subroutine. If the
subroutine has been aliased, the subroutine's original name will be shown,
regardless of what name the caller used.

=item Odd number of arguments for overload::constant

(W overload) The call to overload::constant contained an odd number of
arguments.  The arguments should come in pairs.

=item Odd number of elements in anonymous hash

(W misc) You specified an odd number of elements to initialize a hash,
which is odd, because hashes come in key/value pairs.

=item Odd number of elements in export_lexically

(F) A call to L<builtin/export_lexically> contained an odd number of
arguments.  This is not permitted, because each name must be paired with a
valid reference value.

=item Odd number of elements in hash assignment

(W misc) You specified an odd number of elements to initialize a hash,
which is odd, because hashes come in key/value pairs.

=item Odd number of elements in hash field initialization

(W misc) You specified an odd number of elements to initialise a hash
field of an object.  Hashes are initialised from a list of key/value
pairs so there must be a corresponding value to every key.  The final
missing value will be filled in with undef instead.

=item Offset outside string

(F)(W layer) You tried to do a read/write/send/recv/seek operation
with an offset pointing outside the buffer.  This is difficult to
imagine.  The sole exceptions to this are that zero padding will
take place when going past the end of the string when either
C<sysread()>ing a file, or when seeking past the end of a scalar opened
for I/O (in anticipation of future reads and to imitate the behavior
with real files).

=item Old package separator "'" deprecated

(W deprecated::apostrophe_as_package_separator, syntax) You used the old package
separator "'" in a variable, subroutine or package name. Support for the
old package separator will be removed in Perl 5.42.

=item Old package separator used in string

(W deprecated::apostrophe_as_package_separator, syntax) You used the old package
separator, "'", in a variable named inside a double-quoted string; e.g.,
C<"In $name's house">. This is equivalent to C<"In $name::s house">. If
you meant the former, put a backslash before the apostrophe
(C<"In $name\'s house">).

Support for the old package separator will be removed in Perl 5.42.

=item Only scalar fields can take a :param attribute

(F) You tried to apply the C<:param> attribute to an array or hash field.
Currently this is not permitted.

=item %s() on unopened %s

(W unopened) An I/O operation was attempted on a filehandle that was
never initialized.  You need to do an open(), a sysopen(), or a socket()
call, or call a constructor from the FileHandle package.

=item -%s on unopened filehandle %s

(W unopened) You tried to invoke a file test operator on a filehandle
that isn't open.  Check your control flow.  See also L<perlfunc/-X>.

=item oops: oopsAV

(S internal) An internal warning that the grammar is screwed up.

=item oops: oopsHV

(S internal) An internal warning that the grammar is screwed up.

=item Operand with no preceding operator in regex; marked by S<<-- HERE> in
m/%s/

(F) You wrote something like

 (?[ \p{Digit} \p{Thai} ])

There are two operands, but no operator giving how you want to combine
them.

=item Operation "%s": no method found, %s

(F) An attempt was made to perform an overloaded operation for which no
handler was defined.  While some handlers can be autogenerated in terms
of other handlers, there is no default handler for any operation, unless
the C<fallback> overloading key is specified to be true.  See L<overload>.

=item Operation "%s" returns its argument for non-Unicode code point 0x%X

(S non_unicode) You performed an operation requiring Unicode rules
on a code point that is not in Unicode, so what it should do is not
defined.  Perl has chosen to have it do nothing, and warn you.

If the operation shown is "ToFold", it means that case-insensitive
matching in a regular expression was done on the code point.

If you know what you are doing you can turn off this warning by
C<no warnings 'non_unicode';>.

=item Operation "%s" returns its argument for UTF-16 surrogate U+%X

(S surrogate) You performed an operation requiring Unicode
rules on a Unicode surrogate.  Unicode frowns upon the use
of surrogates for anything but storing strings in UTF-16, but
rules are (reluctantly) defined for the surrogates, and
they are to do nothing for this operation.  Because the use of
surrogates can be dangerous, Perl warns.

If the operation shown is "ToFold", it means that case-insensitive
matching in a regular expression was done on the code point.

If you know what you are doing you can turn off this warning by
C<no warnings 'surrogate';>.

=item Operator or semicolon missing before %s

(S ambiguous) You used a variable or subroutine call where the parser
was expecting an operator.  The parser has assumed you really meant to
use an operator, but this is highly likely to be incorrect.  For
example, if you say "*foo *foo" it will be interpreted as if you said
"*foo * 'foo'".

=item Optional parameter lacks default expression

(F) In a subroutine signature, you wrote something like "$a =", making a
named optional parameter without a default value.  A nameless optional
parameter is permitted to have no default value, but a named one must
have a specific default.  You probably want "$a = undef".

=item "our" variable %s redeclared

(W shadow) You seem to have already declared the same global once before
in the current lexical scope.

=item Out of memory!

(X) The malloc() function returned 0, indicating there was insufficient
remaining memory (or virtual memory) to satisfy the request.  Perl has
no option but to exit immediately.

At least in Unix you may be able to get past this by increasing your
process datasize limits: in csh/tcsh use C<limit> and
C<limit datasize n> (where C<n> is the number of kilobytes) to check
the current limits and change them, and in ksh/bash/zsh use C<ulimit -a>
and C<ulimit -d n>, respectively.

=item Out of memory during %s extend

(X) An attempt was made to extend an array, a list, or a string beyond
the largest possible memory allocation.

=item Out of memory during "large" request for %s

(F) The malloc() function returned 0, indicating there was insufficient
remaining memory (or virtual memory) to satisfy the request.  However,
the request was judged large enough (compile-time default is 64K), so a
possibility to shut down by trapping this error is granted.

=item Out of memory during request for %s

(X)(F) The malloc() function returned 0, indicating there was
insufficient remaining memory (or virtual memory) to satisfy the
request.

The request was judged to be small, so the possibility to trap it
depends on the way perl was compiled.  By default it is not trappable.
However, if compiled for this, Perl may use the contents of C<$^M> as an
emergency pool after die()ing with this message.  In this case the error
is trappable I<once>, and the error message will include the line and file
where the failed request happened.

=item Out of memory during ridiculously large request

(F) You can't allocate more than 2^31+"small amount" bytes.  This error
is most likely to be caused by a typo in the Perl program. e.g.,
C<$arr[time]> instead of C<$arr[$time]>.

=item Out of memory for yacc stack

(F) The yacc parser wanted to grow its stack so it could continue
parsing, but realloc() wouldn't give it more memory, virtual or
otherwise.

=item '.' outside of string in pack

(F) The argument to a '.' in your template tried to move the working
position to before the start of the packed string being built.

=item '@' outside of string in unpack

(F) You had a template that specified an absolute position outside
the string being unpacked.  See L<perlfunc/pack>.

=item '@' outside of string with malformed UTF-8 in unpack

(F) You had a template that specified an absolute position outside
the string being unpacked.  The string being unpacked was also invalid
UTF-8.  See L<perlfunc/pack>.

=item overload arg '%s' is invalid

(W overload) The L<overload> pragma was passed an argument it did not
recognize.  Did you mistype an operator?

=item Overloaded dereference did not return a reference

(F) An object with an overloaded dereference operator was dereferenced,
but the overloaded operation did not return a reference.  See
L<overload>.

=item Overloaded qr did not return a REGEXP

(F) An object with a C<qr> overload was used as part of a match, but the
overloaded operation didn't return a compiled regexp.  See L<overload>.

=item %s package attribute may clash with future reserved word: %s

(W reserved) A lowercase attribute name was used that had a
package-specific handler.  That name might have a meaning to Perl itself
some day, even though it doesn't yet.  Perhaps you should use a
mixed-case attribute name, instead.  See L<attributes>.

=item pack/unpack repeat count overflow

(F) You can't specify a repeat count so large that it overflows your
signed integers.  See L<perlfunc/pack>.

=item page overflow

(W io) A single call to write() produced more lines than can fit on a
page.  See L<perlform>.

=item panic: %s

(P) An internal error.

=item panic: attempt to call %s in %s

(P) One of the file test operators entered a code branch that calls
an ACL related-function, but that function is not available on this
platform.  Earlier checks mean that it should not be possible to
enter this branch on this platform.

=item panic: child pseudo-process was never scheduled

(P) A child pseudo-process in the ithreads implementation on Windows
was not scheduled within the time period allowed and therefore was not
able to initialize properly.

=item panic: ck_grep, type=%u

(P) Failed an internal consistency check trying to compile a grep.

=item panic: corrupt saved stack index %ld

(P) The savestack was requested to restore more localized values than
there are in the savestack.

=item panic: del_backref

(P) Failed an internal consistency check while trying to reset a weak
reference.

=item panic: fold_constants JMPENV_PUSH returned %d

(P) While attempting folding constants an exception other than an C<eval>
failure was caught.

=item panic: frexp: %f

(P) The library function frexp() failed, making printf("%f") impossible.

=item panic: goto, type=%u, ix=%ld

(P) We popped the context stack to a context with the specified label,
and then discovered it wasn't a context we know how to do a goto in.

=item panic: gp_free failed to free glob pointer

(P) The internal routine used to clear a typeglob's entries tried
repeatedly, but each time something re-created entries in the glob.
Most likely the glob contains an object with a reference back to
the glob and a destructor that adds a new object to the glob.

=item panic: INTERPCASEMOD, %s

(P) The lexer got into a bad state at a case modifier.

=item panic: INTERPCONCAT, %s

(P) The lexer got into a bad state parsing a string with brackets.

=item panic: kid popen errno read

(F) A forked child returned an incomprehensible message about its errno.

=item panic: leave_scope inconsistency %u

(P) The savestack probably got out of sync.  At least, there was an
invalid enum on the top of it.

=item panic: magic_killbackrefs

(P) Failed an internal consistency check while trying to reset all weak
references to an object.

=item panic: malloc, %s

(P) Something requested a negative number of bytes of malloc.

=item panic: memory wrap

(P) Something tried to allocate either more memory than possible or a
negative amount.

=item panic: newFORLOOP, %s

(P) The parser failed an internal consistency check while trying to parse
a C<foreach> loop.

=item panic: pad_alloc, %p!=%p

(P) The compiler got confused about which scratch pad it was allocating
and freeing temporaries and lexicals from.

=item panic: pad_free curpad, %p!=%p

(P) The compiler got confused about which scratch pad it was allocating
and freeing temporaries and lexicals from.

=item panic: pad_free po

(P) A zero scratch pad offset was detected internally.  An attempt was
made to free a target that had not been allocated to begin with.

=item panic: pad_reset curpad, %p!=%p

(P) The compiler got confused about which scratch pad it was allocating
and freeing temporaries and lexicals from.

=item panic: pad_sv po

(P) A zero scratch pad offset was detected internally.  Most likely
an operator needed a target but that target had not been allocated
for whatever reason.

=item panic: pad_swipe curpad, %p!=%p

(P) The compiler got confused about which scratch pad it was allocating
and freeing temporaries and lexicals from.

=item panic: pad_swipe po

(P) An invalid scratch pad offset was detected internally.

=item panic: pp_iter, type=%u

(P) The foreach iterator got called in a non-loop context frame.

=item panic: pp_match%s

(P) The internal pp_match() routine was called with invalid operational
data.

=item panic: realloc, %s

(P) Something requested a negative number of bytes of realloc.

=item panic: reference miscount on nsv in sv_replace() (%d != 1)

(P) The internal sv_replace() function was handed a new SV with a
reference count other than 1.

=item panic: restartop in %s

(P) Some internal routine requested a goto (or something like it), and
didn't supply the destination.

=item panic: return, type=%u

(P) We popped the context stack to a subroutine or eval context, and
then discovered it wasn't a subroutine or eval context.

=item panic: scan_num, %s

(P) scan_num() got called on something that wasn't a number.

=item panic: Sequence (?{...}): no code block found in regex m/%s/

(P) While compiling a pattern that has embedded (?{}) or (??{}) code
blocks, perl couldn't locate the code block that should have already been
seen and compiled by perl before control passed to the regex compiler.

=item panic: sv_chop %s

(P) The sv_chop() routine was passed a position that is not within the
scalar's string buffer.

=item panic: sv_insert, midend=%p, bigend=%p

(P) The sv_insert() routine was told to remove more string than there
was string.

=item panic: top_env

(P) The compiler attempted to do a goto, or something weird like that.

=item panic: unexpected constant lvalue entersub entry via type/targ %d:%d

(P) When compiling a subroutine call in lvalue context, Perl failed an
internal consistency check.  It encountered a malformed op tree.

=item panic: unimplemented op %s (#%d) called

(P) The compiler is screwed up and attempted to use an op that isn't
permitted at run time.

=item panic: unknown OA_*: %x

(P) The internal routine that handles arguments to C<&CORE::foo()>
subroutine calls was unable to determine what type of arguments
were expected.

=item panic: utf16_to_utf8: odd bytelen

(P) Something tried to call utf16_to_utf8 with an odd (as opposed
to even) byte length.

=item panic: utf16_to_utf8_reversed: odd bytelen

(P) Something tried to call utf16_to_utf8_reversed with an odd (as opposed
to even) byte length.

=item panic: yylex, %s

(P) The lexer got into a bad state while processing a case modifier.

=item Parentheses missing around "%s" list

(W parenthesis) You said something like

    my $foo, $bar = @_;

when you meant

    my ($foo, $bar) = @_;

Remember that "my", "our", "local" and "state" bind tighter than comma.

=item Parsing code internal error (%s)

(F) Parsing code supplied by an extension violated the parser's API in
a detectable way.

=item Pattern subroutine nesting without pos change exceeded limit in regex

(F) You used a pattern that uses too many nested subpattern calls without
consuming any text.  Restructure the pattern so text is consumed before
the nesting limit is exceeded.

=item C<-p> destination: %s

(F) An error occurred during the implicit output invoked by the C<-p>
command-line switch.  (This output goes to STDOUT unless you've
redirected it with select().)

=item Perl API version %s of %s does not match %s

(F) The XS module in question was compiled against a different incompatible
version of Perl than the one that has loaded the XS module.

=item Perl folding rules are not up-to-date for 0x%X; please use the perlbug
utility to report; in regex; marked by S<<-- HERE> in m/%s/

(S regexp) You used a regular expression with case-insensitive matching,
and there is a bug in Perl in which the built-in regular expression
folding rules are not accurate.  This may lead to incorrect results.
Please report this as a bug to L<https://github.com/Perl/perl5/issues/new/choose>.

=item Perl_my_%s() not available

(F) Your platform has very uncommon byte-order and integer size,
so it was not possible to set up some or all fixed-width byte-order
conversion functions.  This is only a problem when you're using the
'<' or '>' modifiers in (un)pack templates.  See L<perlfunc/pack>.

=item Perl %s required (did you mean %s?)--this is only %s, stopped

(F) The code you are trying to run has asked for a newer version of
Perl than you are running.  Perhaps C<use 5.10> was written instead
of C<use 5.010> or C<use v5.10>.  Without the leading C<v>, the number is
interpreted as a decimal, with every three digits after the
decimal point representing a part of the version number.  So 5.10
is equivalent to v5.100.

=item Perl %s required--this is only %s, stopped

(F) The module in question uses features of a version of Perl more
recent than the currently running version.  How long has it been since
you upgraded, anyway?  See L<perlfunc/require>.

=item PERL_SH_DIR too long

(F) An error peculiar to OS/2.  PERL_SH_DIR is the directory to find the
C<sh>-shell in.  See "PERL_SH_DIR" in L<perlos2>.

=item PERL_SIGNALS illegal: "%s"

(X) See L<perlrun/PERL_SIGNALS> for legal values.

=item Perls since %s too modern--this is %s, stopped

(F) The code you are trying to run claims it will not run
on the version of Perl you are using because it is too new.
Maybe the code needs to be updated, or maybe it is simply
wrong and the version check should just be removed.

=item perl: warning: Non hex character in '$ENV{PERL_HASH_SEED}', seed only partially set

(S) PERL_HASH_SEED should match /^\s*(?:0x)?[0-9a-fA-F]+\s*\z/ but it
contained a non hex character.  This could mean you are not using the
hash seed you think you are.

=item perl: warning: Setting locale failed.

(S) The whole warning message will look something like:

	perl: warning: Setting locale failed.
	perl: warning: Please check that your locale settings:
	        LC_ALL = "En_US",
	        LANG = (unset)
	    are supported and installed on your system.
	perl: warning: Falling back to the standard locale ("C").

Exactly what were the failed locale settings varies.  In the above the
settings were that the LC_ALL was "En_US" and the LANG had no value.
This error means that Perl detected that you and/or your operating
system supplier and/or system administrator have set up the so-called
locale system but Perl could not use those settings.  This was not
dead serious, fortunately: there is a "default locale" called "C" that
Perl can and will use, and the script will be run.  Before you really
fix the problem, however, you will get the same error message each
time you run Perl.  How to really fix the problem can be found in
L<perllocale> section B<LOCALE PROBLEMS>.

=item perl: warning: strange setting in '$ENV{PERL_PERTURB_KEYS}': '%s'

(S) Perl was run with the environment variable PERL_PERTURB_KEYS defined
but containing an unexpected value.  The legal values of this setting
are as follows.

  Numeric | String        | Result
  --------+---------------+-----------------------------------------
  0       | NO            | Disables key traversal randomization
  1       | RANDOM        | Enables full key traversal randomization
  2       | DETERMINISTIC | Enables repeatable key traversal
          |               | randomization

Both numeric and string values are accepted, but note that string values are
case sensitive.  The default for this setting is "RANDOM" or 1.

=item pid %x not a child

(W exec) A warning peculiar to VMS.  Waitpid() was asked to wait for a
process which isn't a subprocess of the current process.  While this is
fine from VMS' perspective, it's probably not what you intended.

=item 'P' must have an explicit size in unpack

(F) The unpack format P must have an explicit size, not "*".

=item POSIX class [:%s:] unknown in regex; marked by S<<-- HERE> in m/%s/

(F) The class in the character class [: :] syntax is unknown.  The S<<-- HERE>
shows whereabouts in the regular expression the problem was discovered.
Note that the POSIX character classes do B<not> have the C<is> prefix
the corresponding C interfaces have: in other words, it's C<[[:print:]]>,
not C<isprint>.  See L<perlre>.

=item POSIX getpgrp can't take an argument

(F) Your system has POSIX getpgrp(), which takes no argument, unlike
the BSD version, which takes a pid.

=item POSIX syntax [%c %c] belongs inside character classes%s in regex; marked by
S<<-- HERE> in m/%s/

(W regexp) Perl thinks that you intended to write a POSIX character
class, but didn't use enough brackets.  These POSIX class constructs [:
:], [= =], and [. .]  go I<inside> character classes, the [] are part of
the construct, for example: C<qr/[012[:alpha:]345]/>.  What the regular
expression pattern compiled to is probably not what you were intending.
For example, C<qr/[:alpha:]/> compiles to a regular bracketed character
class consisting of the four characters C<":">,  C<"a">,  C<"l">,
C<"h">, and C<"p">.  To specify the POSIX class, it should have been
written C<qr/[[:alpha:]]/>.

Note that [= =] and [. .] are not currently
implemented; they are simply placeholders for future extensions and
will cause fatal errors.  The S<<-- HERE> shows whereabouts in the regular
expression the problem was discovered.  See L<perlre>.

If the specification of the class was not completely valid, the message
indicates that.

=item POSIX syntax [. .] is reserved for future extensions in regex; marked by 
S<<-- HERE> in m/%s/

(F) Within regular expression character classes ([]) the syntax beginning
with "[." and ending with ".]" is reserved for future extensions.  If you
need to represent those character sequences inside a regular expression
character class, just quote the square brackets with the backslash: "\[."
and ".\]".  The S<<-- HERE> shows whereabouts in the regular expression the
problem was discovered.  See L<perlre>.

=item POSIX syntax [= =] is reserved for future extensions in regex; marked by 
S<<-- HERE> in m/%s/

(F) Within regular expression character classes ([]) the syntax beginning
with "[=" and ending with "=]" is reserved for future extensions.  If you
need to represent those character sequences inside a regular expression
character class, just quote the square brackets with the backslash: "\[="
and "=\]".  The S<<-- HERE> shows whereabouts in the regular expression the
problem was discovered.  See L<perlre>.

=item Possible attempt to put comments in qw() list

(W qw) qw() lists contain items separated by whitespace; as with literal
strings, comment characters are not ignored, but are instead treated as
literal data.  (You may have used different delimiters than the
parentheses shown here; braces are also frequently used.)

You probably wrote something like this:

    @list = qw(
	a # a comment
        b # another comment
    );

when you should have written this:

    @list = qw(
	a
        b
    );

If you really want comments, build your list the
old-fashioned way, with quotes and commas:

    @list = (
        'a',    # a comment
        'b',    # another comment
    );

=item Possible attempt to separate words with commas

(W qw) qw() lists contain items separated by whitespace; therefore
commas aren't needed to separate the items.  (You may have used
different delimiters than the parentheses shown here; braces are also
frequently used.)

You probably wrote something like this:

    qw! a, b, c !;

which puts literal commas into some of the list items.  Write it without
commas if you don't want them to appear in your data:

    qw! a b c !;

=item Possible memory corruption: %s overflowed 3rd argument

(F) An ioctl() or fcntl() returned more than Perl was bargaining for.
Perl guesses a reasonable buffer size, but puts a sentinel byte at the
end of the buffer just in case.  This sentinel byte got clobbered, and
Perl assumes that memory is now corrupted.  See L<perlfunc/ioctl>.

=item Possible precedence issue with control flow operator

(W syntax) There is a possible problem with the mixing of a control
flow operator (e.g. C<return>) and a low-precedence operator like
C<or>.  Consider:

    sub { return $a or $b; }

This is parsed as:

    sub { (return $a) or $b; }

Which is effectively just:

    sub { return $a; }

Either use parentheses or the high-precedence variant of the operator.

Note this may be also triggered for constructs like:

    sub { 1 if die; }

=item Possible precedence problem on bitwise %s operator

(W precedence) Your program uses a bitwise logical operator in conjunction
with a numeric comparison operator, like this :

    if ($x & $y == 0) { ... }

This expression is actually equivalent to C<$x & ($y == 0)>, due to the
higher precedence of C<==>.  This is probably not what you want.  (If you
really meant to write this, disable the warning, or, better, put the
parentheses explicitly and write C<$x & ($y == 0)>).

=item Possible unintended interpolation of $\ in regex

(W ambiguous) You said something like C<m/$\/> in a regex.
The regex C<m/foo$\s+bar/m> translates to: match the word 'foo', the output
record separator (see L<perlvar/$\>) and the letter 's' (one time or more)
followed by the word 'bar'.

If this is what you intended then you can silence the warning by using 
C<m/${\}/> (for example: C<m/foo${\}s+bar/>).

If instead you intended to match the word 'foo' at the end of the line
followed by whitespace and the word 'bar' on the next line then you can use
C<m/$(?)\/> (for example: C<m/foo$(?)\s+bar/>).

=item Possible unintended interpolation of %s in string

(W ambiguous) You said something like '@foo' in a double-quoted string
but there was no array C<@foo> in scope at the time.  If you wanted a
literal @foo, then write it as \@foo; otherwise find out what happened
to the array you apparently lost track of.

=item Precedence problem: open %s should be open(%s)

(S precedence) The old irregular construct

    open FOO || die;

is now misinterpreted as

    open(FOO || die);

because of the strict regularization of Perl 5's grammar into unary and
list operators.  (The old open was a little of both.)  You must put
parentheses around the filehandle, or use the new "or" operator instead
of "||".

=item Premature end of script headers

See L</500 Server error>.

=item printf() on closed filehandle %s

(W closed) The filehandle you're writing to got itself closed sometime
before now.  Check your control flow.

=item print() on closed filehandle %s

(W closed) The filehandle you're printing on got itself closed sometime
before now.  Check your control flow.

=item Process terminated by SIG%s

(W) This is a standard message issued by OS/2 applications, while *nix
applications die in silence.  It is considered a feature of the OS/2
port.  One can easily disable this by appropriate sighandlers, see
L<perlipc/"Signals">.  See also "Process terminated by SIGTERM/SIGINT"
in L<perlos2>.

=item Prototype after '%c' for %s : %s

(W illegalproto) A character follows % or @ in a prototype.  This is
useless, since % and @ gobble the rest of the subroutine arguments.

=item Prototype mismatch: %s vs %s

(S prototype) The subroutine being declared or defined had previously been
declared or defined with a different function prototype.

=item Prototype not terminated

(F) You've omitted the closing parenthesis in a function prototype
definition.

=item Prototype '%s' overridden by attribute 'prototype(%s)' in %s

(W prototype) A prototype was declared in both the parentheses after
the sub name and via the prototype attribute.  The prototype in
parentheses is useless, since it will be replaced by the prototype
from the attribute before it's ever used.

=item %s on BEGIN block ignored

(W syntax) C<BEGIN> blocks are executed immediately after they are parsed
and then thrown away. Any prototypes or attributes are therefore
meaningless and are ignored. You should remove them from the C<BEGIN> block.
Note this also means you cannot create a constant called C<BEGIN>.

=item Quantifier follows nothing in regex; marked by S<<-- HERE> in m/%s/

(F) You started a regular expression with a quantifier.  Backslash it if
you meant it literally.  The S<<-- HERE> shows whereabouts in the regular
expression the problem was discovered.  See L<perlre>.

=item Quantifier in {,} bigger than %d in regex; marked by S<<-- HERE> in m/%s/

(F) There is currently a limit to the size of the min and max values of
the {min,max} construct.  The S<<-- HERE> shows whereabouts in the regular
expression the problem was discovered.  See L<perlre>.

=item Quantifier {n,m} with n > m can't match in regex

=item Quantifier {n,m} with n > m can't match in regex; marked by
S<<-- HERE> in m/%s/

(W regexp) Minima should be less than or equal to maxima.  If you really
want your regexp to match something 0 times, just put {0}.

=item Quantifier unexpected on zero-length expression in regex m/%s/

(W regexp) You applied a regular expression quantifier in a place where
it makes no sense, such as on a zero-width assertion.  Try putting the
quantifier inside the assertion instead.  For example, the way to match
"abc" provided that it is followed by three repetitions of "xyz" is
C</abc(?=(?:xyz){3})/>, not C</abc(?=xyz){3}/>.

=item Range iterator outside integer range

(F) One (or both) of the numeric arguments to the range operator ".."
are outside the range which can be represented by integers internally.
One possible workaround is to force Perl to use magical string increment
by prepending "0" to your numbers.

=item Ranges of ASCII printables should be some subset of "0-9", "A-Z", or
"a-z" in regex; marked by S<<-- HERE> in m/%s/

(W regexp) (only under C<S<use re 'strict'>> or within C<(?[...])>)

Stricter rules help to find typos and other errors.  Perhaps you didn't
even intend a range here, if the C<"-"> was meant to be some other
character, or should have been escaped (like C<"\-">).  If you did
intend a range, the one that was used is not portable between ASCII and
EBCDIC platforms, and doesn't have an obvious meaning to a casual
reader.

 [3-7]    # OK; Obvious and portable
 [d-g]    # OK; Obvious and portable
 [A-Y]    # OK; Obvious and portable
 [A-z]    # WRONG; Not portable; not clear what is meant
 [a-Z]    # WRONG; Not portable; not clear what is meant
 [%-.]    # WRONG; Not portable; not clear what is meant
 [\x41-Z] # WRONG; Not portable; not obvious to non-geek

(You can force portability by specifying a Unicode range, which means that
the endpoints are specified by
L<C<\N{...}>|perlrecharclass/Character Ranges>, but the meaning may
still not be obvious.)
The stricter rules require that ranges that start or stop with an ASCII
character that is not a control have all their endpoints be the literal
character, and not some escape sequence (like C<"\x41">), and the ranges
must be all digits, or all uppercase letters, or all lowercase letters.

=item Ranges of digits should be from the same group in regex; marked by
S<<-- HERE> in m/%s/

(W regexp) (only under C<S<use re 'strict'>> or within C<(?[...])>)

Stricter rules help to find typos and other errors.  You included a
range, and at least one of the end points is a decimal digit.  Under the
stricter rules, when this happens, both end points should be digits in
the same group of 10 consecutive digits.

=item readdir() attempted on invalid dirhandle %s

(W io) The dirhandle you're reading from is either closed or not really
a dirhandle.  Check your control flow.

=item readline() on closed filehandle %s

(W closed) The filehandle you're reading from got itself closed sometime
before now.  Check your control flow.

=item readline() on unopened filehandle %s

(W unopened) The filehandle you're reading from was never opened.  Check your
control flow.

=item read() on closed filehandle %s

(W closed) You tried to read from a closed filehandle.

=item read() on unopened filehandle %s

(W unopened) You tried to read from a filehandle that was never opened.

=item realloc() of freed memory ignored

(S malloc) An internal routine called realloc() on something that had
already been freed.

=item Recompile perl with B<-D>DEBUGGING to use B<-D> switch

(S debugging) You can't use the B<-D> option unless the code to produce
the desired output is compiled into Perl, which entails some overhead,
which is why it's currently left out of your copy.

=item Recursive call to Perl_load_module in PerlIO_find_layer

(P) It is currently not permitted to load modules when creating
a filehandle inside an %INC hook.  This can happen with C<open my
$fh, '<', \$scalar>, which implicitly loads PerlIO::scalar.  Try
loading PerlIO::scalar explicitly first.

=item Recursive inheritance detected in package '%s'

(F) While calculating the method resolution order (MRO) of a package, Perl
believes it found an infinite loop in the C<@ISA> hierarchy.  This is a
crude check that bails out after 100 levels of C<@ISA> depth.

=item Redundant argument in %s

(W redundant) You called a function with more arguments than other
arguments you supplied indicated would be needed.  Currently only
emitted when a printf-type format required fewer arguments than were
supplied, but might be used in the future for e.g. L<perlfunc/pack>.

=item refcnt_dec: fd %d%s

=item refcnt: fd %d%s

=item refcnt_inc: fd %d%s

(P) Perl's I/O implementation failed an internal consistency check.  If
you see this message, something is very wrong.

=item Reference found where even-sized list expected

(W misc) You gave a single reference where Perl was expecting a list
with an even number of elements (for assignment to a hash).  This
usually means that you used the anon hash constructor when you meant
to use parens.  In any case, a hash requires key/value B<pairs>.

    %hash = { one => 1, two => 2, };	# WRONG
    %hash = [ qw/ an anon array / ];	# WRONG
    %hash = ( one => 1, two => 2, );	# right
    %hash = qw( one 1 two 2 );			# also fine

=item Reference is already weak

(W misc) You have attempted to weaken a reference that is already weak.
Doing so has no effect.

=item Reference is not weak

(W misc) You have attempted to unweaken a reference that is not weak.
Doing so has no effect.

=item Reference to invalid group 0 in regex; marked by S<<-- HERE> in m/%s/

(F) You used C<\g0> or similar in a regular expression.  You may refer
to capturing parentheses only with strictly positive integers
(normal backreferences) or with strictly negative integers (relative
backreferences).  Using 0 does not make sense.

=item Reference to nonexistent group in regex; marked by S<<-- HERE> in
m/%s/

(F) You used something like C<\7> in your regular expression, but there are
not at least seven sets of capturing parentheses in the expression.  If
you wanted to have the character with ordinal 7 inserted into the regular
expression, prepend zeroes to make it three digits long: C<\007>

The S<<-- HERE> shows whereabouts in the regular expression the problem was
discovered.

=item Reference to nonexistent named group in regex; marked by S<<-- HERE>
in m/%s/

(F) You used something like C<\k'NAME'> or C<< \k<NAME> >> in your regular
expression, but there is no corresponding named capturing parentheses
such as C<(?'NAME'...)> or C<< (?<NAME>...) >>.  Check if the name has been
spelled correctly both in the backreference and the declaration.

The S<<-- HERE> shows whereabouts in the regular expression the problem was
discovered.

=item Reference to nonexistent or unclosed group in regex; marked by
S<<-- HERE> in m/%s/

(F) You used something like C<\g{-7}> in your regular expression, but there
are not at least seven sets of closed capturing parentheses in the
expression before where the C<\g{-7}> was located.

The S<<-- HERE> shows whereabouts in the regular expression the problem was
discovered.

=item regexp memory corruption

(P) The regular expression engine got confused by what the regular
expression compiler gave it.

=item Regexp modifier "/%c" may appear a maximum of twice

=item Regexp modifier "%c" may appear a maximum of twice in regex; marked
by S<<-- HERE> in m/%s/

(F) The regular expression pattern had too many occurrences
of the specified modifier.  Remove the extraneous ones.

=item Regexp modifier "%c" may not appear after the "-" in regex; marked by <-- 
HERE in m/%s/

(F) Turning off the given modifier has the side effect of turning on
another one.  Perl currently doesn't allow this.  Reword the regular
expression to use the modifier you want to turn on (and place it before
the minus), instead of the one you want to turn off.

=item Regexp modifier "/%c" may not appear twice

=item Regexp modifier "%c" may not appear twice in regex; marked by <--
HERE in m/%s/

(F) The regular expression pattern had too many occurrences
of the specified modifier.  Remove the extraneous ones.

=item Regexp modifiers "/%c" and "/%c" are mutually exclusive

=item Regexp modifiers "%c" and "%c" are mutually exclusive in regex;
marked by S<<-- HERE> in m/%s/

(F) The regular expression pattern had more than one of these
mutually exclusive modifiers.  Retain only the modifier that is
supposed to be there.

=item Regexp out of space in regex m/%s/

(P) A "can't happen" error, because safemalloc() should have caught it
earlier.

=item Repeated format line will never terminate (~~ and @#)

(F) Your format contains the ~~ repeat-until-blank sequence and a
numeric field that will never go blank so that the repetition never
terminates.  You might use ^# instead.  See L<perlform>.

=item Replacement list is longer than search list

(W misc) You have used a replacement list that is longer than the
search list.  So the additional elements in the replacement list
are meaningless.

=item Required parameter '%s' is missing for %s constructor

(F) You called the constructor for a class that has a required named
parameter, but did not pass that parameter at all.

=item '(*%s' requires a terminating ':' in regex; marked by <-- HERE in m/%s/

(F) You used a construct that needs a colon and pattern argument.
Supply these or check that you are using the right construct.

=item '%s' resolved to '\o{%s}%d'

As of Perl 5.32, this message is no longer generated.  Instead, see
L</Non-octal character '%c' terminates \o early.  Resolved as "%s">.
(W misc, regexp)  You wrote something like C<\08>, or C<\179> in a
double-quotish string.  All but the last digit is treated as a single
character, specified in octal.  The last digit is the next character in
the string.  To tell Perl that this is indeed what you want, you can use
the C<\o{ }> syntax, or use exactly three digits to specify the octal
for the character.

=item Reversed %s= operator

(W syntax) You wrote your assignment operator backwards.  The = must
always come last, to avoid ambiguity with subsequent unary operators.

=item rewinddir() attempted on invalid dirhandle %s

(W io) The dirhandle you tried to do a rewinddir() on is either closed
or not really a dirhandle.  Check your control flow.

=item Scalars leaked: %d

(S internal) Something went wrong in Perl's internal bookkeeping
of scalars: not all scalar variables were deallocated by the time
Perl exited.  What this usually indicates is a memory leak, which
is of course bad, especially if the Perl program is intended to be
long-running.

=item Scalar value @%s[%s] better written as $%s[%s]

(W syntax) You've used an array slice (indicated by @) to select a
single element of an array.  Generally it's better to ask for a scalar
value (indicated by $).  The difference is that C<$foo[&bar]> always
behaves like a scalar, both when assigning to it and when evaluating its
argument, while C<@foo[&bar]> behaves like a list when you assign to it,
and provides a list context to its subscript, which can do weird things
if you're expecting only one subscript.

On the other hand, if you were actually hoping to treat the array
element as a list, you need to look into how references work, because
Perl will not magically convert between scalars and lists for you.  See
L<perlref>.

=item Scalar value @%s{%s} better written as $%s{%s}

(W syntax) You've used a hash slice (indicated by @) to select a single
element of a hash.  Generally it's better to ask for a scalar value
(indicated by $).  The difference is that C<$foo{&bar}> always behaves
like a scalar, both when assigning to it and when evaluating its
argument, while C<@foo{&bar}> behaves like a list when you assign to it,
and provides a list context to its subscript, which can do weird things
if you're expecting only one subscript.

On the other hand, if you were actually hoping to treat the hash element
as a list, you need to look into how references work, because Perl will
not magically convert between scalars and lists for you.  See
L<perlref>.

=item Search pattern not terminated

(F) The lexer couldn't find the final delimiter of a // or m{}
construct.  Remember that bracketing delimiters count nesting level.
Missing the leading C<$> from a variable C<$m> may cause this error.

Note that since Perl 5.10.0 a // can also be the I<defined-or>
construct, not just the empty search pattern.  Therefore code written
in Perl 5.10.0 or later that uses the // as the I<defined-or> can be
misparsed by pre-5.10.0 Perls as a non-terminated search pattern.

=item seekdir() attempted on invalid dirhandle %s

(W io) The dirhandle you are doing a seekdir() on is either closed or not
really a dirhandle.  Check your control flow.

=item %sseek() on unopened filehandle

(W unopened) You tried to use the seek() or sysseek() function on a
filehandle that was either never opened or has since been closed.

=item select not implemented

(F) This machine doesn't implement the select() system call.

=item Self-ties of arrays and hashes are not supported

(F) Self-ties are of arrays and hashes are not supported in
the current implementation.

=item Semicolon seems to be missing

(W semicolon) A nearby syntax error was probably caused by a missing
semicolon, or possibly some other missing operator, such as a comma.

=item semi-panic: attempt to dup freed string

(S internal) The internal newSVsv() routine was called to duplicate a
scalar that had previously been marked as free.

=item sem%s not implemented

(F) You don't have System V semaphore IPC on your system.

=item send() on closed socket %s

(W closed) The socket you're sending to got itself closed sometime
before now.  Check your control flow.

=item Sequence "\c{" invalid

(F) These three characters may not appear in sequence in a
double-quotish context.  This message is raised only on non-ASCII
platforms (a different error message is output on ASCII ones).  If you
were intending to specify a control character with this sequence, you'll
have to use a different way to specify it.

=item Sequence (? incomplete in regex; marked by S<<-- HERE> in m/%s/

(F) A regular expression ended with an incomplete extension (?.  The
S<<-- HERE> shows whereabouts in the regular expression the problem was
discovered.  See L<perlre>.

=item Sequence (?%c...) not implemented in regex; marked by S<<-- HERE> in
m/%s/

(F) A proposed regular expression extension has the character reserved
but has not yet been written.  The S<<-- HERE> shows whereabouts in the
regular expression the problem was discovered.  See L<perlre>.

=item Sequence (?%s...) not recognized in regex; marked by S<<-- HERE> in
m/%s/

(F) You used a regular expression extension that doesn't make sense.
The S<<-- HERE> shows whereabouts in the regular expression the problem was
discovered.  This may happen when using the C<(?^...)> construct to tell
Perl to use the default regular expression modifiers, and you
redundantly specify a default modifier.  For other
causes, see L<perlre>.

=item Sequence (?#... not terminated in regex m/%s/

(F) A regular expression comment must be terminated by a closing
parenthesis.  Embedded parentheses aren't allowed.  See
L<perlre>.

=item Sequence (?&... not terminated in regex; marked by S<<-- HERE> in
m/%s/

(F) A named reference of the form C<(?&...)> was missing the final
closing parenthesis after the name.  The S<<-- HERE> shows whereabouts
in the regular expression the problem was discovered.

=item Sequence (?%c... not terminated in regex; marked by S<<-- HERE>
in m/%s/

(F) A named group of the form C<(?'...')> or C<< (?<...>) >> was missing the final
closing quote or angle bracket.  The S<<-- HERE> shows whereabouts in the
regular expression the problem was discovered.

=item Sequence (%s... not terminated in regex; marked by S<<-- HERE>
in m/%s/

(F) A lookahead assertion C<(?=...)> or C<(?!...)> or lookbehind
assertion C<< (?<=...) >> or C<< (?<!...) >> was missing the final
closing parenthesis.  The S<<-- HERE> shows whereabouts in the
regular expression the problem was discovered.

=item Sequence (?(%c... not terminated in regex; marked by S<<-- HERE>
in m/%s/

(F) A named reference of the form C<(?('...')...)> or C<< (?(<...>)...) >> was
missing the final closing quote or angle bracket after the name.  The
S<<-- HERE> shows whereabouts in the regular expression the problem was
discovered.

=item Sequence (?... not terminated in regex; marked by S<<-- HERE> in
m/%s/

(F) There was no matching closing parenthesis for the '('.  The
S<<-- HERE> shows whereabouts in the regular expression the problem was
discovered.

=item Sequence \%s... not terminated in regex; marked by S<<-- HERE> in
m/%s/

(F) The regular expression expects a mandatory argument following the escape
sequence and this has been omitted or incorrectly written.

=item Sequence (?{...}) not terminated with ')'

(F) The end of the perl code contained within the {...} must be
followed immediately by a ')'.

=item Sequence (?PE<gt>... not terminated in regex; marked by S<<-- HERE> in m/%s/

(F) A named reference of the form C<(?PE<gt>...)> was missing the final
closing parenthesis after the name.  The S<<-- HERE> shows whereabouts
in the regular expression the problem was discovered.

=item Sequence (?PE<lt>... not terminated in regex; marked by S<<-- HERE> in m/%s/

(F) A named group of the form C<(?PE<lt>...E<gt>')> was missing the final
closing angle bracket.  The S<<-- HERE> shows whereabouts in the
regular expression the problem was discovered.

=item Sequence ?P=... not terminated in regex; marked by S<<-- HERE> in
m/%s/

(F) A named reference of the form C<(?P=...)> was missing the final
closing parenthesis after the name.  The S<<-- HERE> shows whereabouts
in the regular expression the problem was discovered.

=item Sequence (?R) not terminated in regex m/%s/

(F) An C<(?R)> or C<(?0)> sequence in a regular expression was missing the
final parenthesis.

=item Z<>500 Server error

(A) This is the error message generally seen in a browser window
when trying to run a CGI program (including SSI) over the web.  The
actual error text varies widely from server to server.  The most
frequently-seen variants are "500 Server error", "Method (something)
not permitted", "Document contains no data", "Premature end of script
headers", and "Did not produce a valid header".

B<This is a CGI error, not a Perl error>.

You need to make sure your script is executable, is accessible by
the user CGI is running the script under (which is probably not the
user account you tested it under), does not rely on any environment
variables (like PATH) from the user it isn't running under, and isn't
in a location where the CGI server can't find it, basically, more or
less.  Please see the following for more information:

	https://www.perl.org/CGI_MetaFAQ.html
	http://www.htmlhelp.org/faq/cgifaq.html
	http://www.w3.org/Security/Faq/

You should also look at L<perlfaq9>.

=item setegid() not implemented

(F) You tried to assign to C<$)>, and your operating system doesn't
support the setegid() system call (or equivalent), or at least Configure
didn't think so.

=item seteuid() not implemented

(F) You tried to assign to C<< $> >>, and your operating system doesn't
support the seteuid() system call (or equivalent), or at least Configure
didn't think so.

=item setpgrp can't take arguments

(F) Your system has the setpgrp() from BSD 4.2, which takes no
arguments, unlike POSIX setpgid(), which takes a process ID and process
group ID.

=item setrgid() not implemented

(F) You tried to assign to C<$(>, and your operating system doesn't
support the setrgid() system call (or equivalent), or at least Configure
didn't think so.

=item setruid() not implemented

(F) You tried to assign to C<$<>, and your operating system doesn't
support the setruid() system call (or equivalent), or at least Configure
didn't think so.

=item setsockopt() on closed socket %s

(W closed) You tried to set a socket option on a closed socket.  Did you
forget to check the return value of your socket() call?  See
L<perlfunc/setsockopt>.

=item Setting $/ to a reference to %s is forbidden

(F) You assigned a reference to a scalar to C<$/> where the referenced item is
not a positive integer.  In older perls this B<appeared> to work the same as
setting it to C<undef> but was in fact internally different, less efficient
and with very bad luck could have resulted in your file being split by a
stringified form of the reference.

In Perl 5.20.0 this was changed so that it would be B<exactly> the same as
setting C<$/> to undef, with the exception that this warning would be thrown.

You are recommended to change your code to set C<$/> to C<undef> explicitly if
you wish to slurp the file.  As of Perl 5.28 assigning C<$/> to a reference
to an integer which isn't positive is a fatal error.

=item Setting $/ to %s reference is forbidden

(F) You tried to assign a reference to a non integer to C<$/>.  In older
Perls this would have behaved similarly to setting it to a reference to
a positive integer, where the integer was the address of the reference.
As of Perl 5.20.0 this is a fatal error, to allow future versions of Perl
to use non-integer refs for more interesting purposes.

=item shm%s not implemented

(F) You don't have System V shared memory IPC on your system.

=item !=~ should be !~

(W syntax) The non-matching operator is !~, not !=~.  !=~ will be
interpreted as the != (numeric not equal) and ~ (1's complement)
operators: probably not what you intended.

=item /%s/ should probably be written as "%s"

(W syntax) You have used a pattern where Perl expected to find a string,
as in the first argument to C<join>.  Perl will treat the true or false
result of matching the pattern against $_ as the string, which is
probably not what you had in mind.

=item shutdown() on closed socket %s

(W closed) You tried to do a shutdown on a closed socket.  Seems a bit
superfluous.

=item SIG%s handler "%s" not defined

(W signal) The signal handler named in %SIG doesn't, in fact, exist.
Perhaps you put it into the wrong package?

=item Slab leaked from cv %p

(S) If you see this message, then something is seriously wrong with the
internal bookkeeping of op trees.  An op tree needed to be freed after
a compilation error, but could not be found, so it was leaked instead.

=item sleep(%u) too large

(W overflow) You called C<sleep> with a number that was larger than
it can reliably handle and C<sleep> probably slept for less time than
requested.

=item Slurpy parameter not last

(F) In a subroutine signature, you put something after a slurpy (array or
hash) parameter.  The slurpy parameter takes all the available arguments,
so there can't be any left to fill later parameters.

=item Smart matching a non-overloaded object breaks encapsulation

(F) You should not use the C<~~> operator on an object that does not
overload it: Perl refuses to use the object's underlying structure
for the smart match.

=item Smartmatch is deprecated

(D deprecated::smartmatch) This warning is emitted if you
use the smartmatch (C<~~>) operator.  This is a deprecated
feature.  Particularly, its behavior is noticed for being
unnecessarily complex and unintuitive, and it will be removed
in Perl 5.42.

=item Sorry, hash keys must be smaller than 2**31 bytes

(F) You tried to create a hash containing a very large key, where "very
large" means that it needs at least 2 gigabytes to store. Unfortunately,
Perl doesn't yet handle such large hash keys. You should
reconsider your design to avoid hashing such a long string directly.

=item sort is now a reserved word

(F) An ancient error message that almost nobody ever runs into anymore.
But before sort was a keyword, people sometimes used it as a filehandle.

=item Source filters apply only to byte streams

(F) You tried to activate a source filter (usually by loading a
source filter module) within a string passed to C<eval>.  This is
not permitted under the C<unicode_eval> feature.  Consider using
C<evalbytes> instead.  See L<feature>.

=item splice() offset past end of array

(W misc) You attempted to specify an offset that was past the end of
the array passed to splice().  Splicing will instead commence at the
end of the array, rather than past it.  If this isn't what you want,
try explicitly pre-extending the array by assigning $#array = $offset.
See L<perlfunc/splice>.

=item Split loop

(P) The split was looping infinitely.  (Obviously, a split shouldn't
iterate more times than there are characters of input, which is what
happened.)  See L<perlfunc/split>.

=item Statement unlikely to be reached

(W exec) You did an exec() with some statement after it other than a
die().  This is almost always an error, because exec() never returns
unless there was a failure.  You probably wanted to use system()
instead, which does return.  To suppress this warning, put the exec() in
a block by itself.

=item "state" subroutine %s can't be in a package

(F) Lexically scoped subroutines aren't in a package, so it doesn't make
sense to try to declare one with a package qualifier on the front.

=item "state %s" used in sort comparison

(W syntax) The package variables $a and $b are used for sort comparisons.
You used $a or $b in as an operand to the C<< <=> >> or C<cmp> operator inside a
sort comparison block, and the variable had earlier been declared as a
lexical variable.  Either qualify the sort variable with the package
name, or rename the lexical variable.

=item "state" variable %s can't be in a package

(F) Lexically scoped variables aren't in a package, so it doesn't make
sense to try to declare one with a package qualifier on the front.  Use
local() if you want to localize a package variable.

=item stat() on unopened filehandle %s

(W unopened) You tried to use the stat() function on a filehandle that
was either never opened or has since been closed.

=item Strings with code points over 0xFF may not be mapped into in-memory file handles

(W utf8) You tried to open a reference to a scalar for read or append
where the scalar contained code points over 0xFF.  In-memory files
model on-disk files and can only contain bytes.

=item Stub found while resolving method "%s" overloading "%s" in package "%s"

(P) Overloading resolution over @ISA tree may be broken by importation
stubs.  Stubs should never be implicitly created, but explicit calls to
C<can> may break this.

=item Subroutine attributes must come before the signature

(F) When subroutine signatures are enabled, any subroutine attributes must
come before the signature. Note that this order was the opposite in
versions 5.22..5.26. So:

    sub foo :lvalue ($a, $b) { ... }  # 5.20 and 5.28 +
    sub foo ($a, $b) :lvalue { ... }  # 5.22 .. 5.26

=item Subroutine "&%s" is not available

(W closure) During compilation, an inner named subroutine or eval is
attempting to capture an outer lexical subroutine that is not currently
available.  This can happen for one of two reasons.  First, the lexical
subroutine may be declared in an outer anonymous subroutine that has
not yet been created.  (Remember that named subs are created at compile
time, while anonymous subs are created at run-time.)  For example,

    sub { my sub a {...} sub f { \&a } }

At the time that f is created, it can't capture the current "a" sub,
since the anonymous subroutine hasn't been created yet.  Conversely, the
following won't give a warning since the anonymous subroutine has by now
been created and is live:

    sub { my sub a {...} eval 'sub f { \&a }' }->();

The second situation is caused by an eval accessing a lexical subroutine
that has gone out of scope, for example,

    sub f {
	my sub a {...}
	sub { eval '\&a' }
    }
    f()->();

Here, when the '\&a' in the eval is being compiled, f() is not currently
being executed, so its &a is not available for capture.

=item "%s" subroutine &%s masks earlier declaration in same %s

(W shadow) A "my" or "state" subroutine has been redeclared in the
current scope or statement, effectively eliminating all access to
the previous instance.  This is almost always a typographical error.
Note that the earlier subroutine will still exist until the end of
the scope or until all closure references to it are destroyed.

=item Subroutine %s redefined

(W redefine) You redefined a subroutine.  To suppress this warning, say

    {
	no warnings 'redefine';
	eval "sub name { ... }";
    }

=item Subroutine "%s" will not stay shared

(W closure) An inner (nested) I<named> subroutine is referencing a "my"
subroutine defined in an outer named subroutine.

When the inner subroutine is called, it will see the value of the outer
subroutine's lexical subroutine as it was before and during the *first*
call to the outer subroutine; in this case, after the first call to the
outer subroutine is complete, the inner and outer subroutines will no
longer share a common value for the lexical subroutine.  In other words,
it will no longer be shared.  This will especially make a difference
if the lexical subroutines accesses lexical variables declared in its
surrounding scope.

This problem can usually be solved by making the inner subroutine
anonymous, using the C<sub {}> syntax.  When inner anonymous subs that
reference lexical subroutines in outer subroutines are created, they
are automatically rebound to the current values of such lexical subs.

=item Substitution loop

(P) The substitution was looping infinitely.  (Obviously, a substitution
shouldn't iterate more times than there are characters of input, which
is what happened.)  See the discussion of substitution in
L<perlop/"Regexp Quote-Like Operators">.

=item Substitution pattern not terminated

(F) The lexer couldn't find the interior delimiter of an s/// or s{}{}
construct.  Remember that bracketing delimiters count nesting level.
Missing the leading C<$> from variable C<$s> may cause this error.

=item Substitution replacement not terminated

(F) The lexer couldn't find the final delimiter of an s/// or s{}{}
construct.  Remember that bracketing delimiters count nesting level.
Missing the leading C<$> from variable C<$s> may cause this error.

=item substr outside of string

(W substr)(F) You tried to reference a substr() that pointed outside of
a string.  That is, the absolute value of the offset was larger than the
length of the string.  See L<perlfunc/substr>.  This warning is fatal if
substr is used in an lvalue context (as the left hand side of an
assignment or as a subroutine argument for example).

=item sv_upgrade from type %d down to type %d

(P) Perl tried to force the upgrade of an SV to a type which was actually
inferior to its current type.

=item Switch (?(condition)... contains too many branches in regex; marked by 
S<<-- HERE> in m/%s/

(F) A (?(condition)if-clause|else-clause) construct can have at most
two branches (the if-clause and the else-clause).  If you want one or
both to contain alternation, such as using C<this|that|other>, enclose
it in clustering parentheses:

    (?(condition)(?:this|that|other)|else-clause)

The S<<-- HERE> shows whereabouts in the regular expression the problem
was discovered.  See L<perlre>.

=item Switch condition not recognized in regex; marked by S<<-- HERE> in
m/%s/

(F) The condition part of a (?(condition)if-clause|else-clause) construct
is not known.  The condition must be one of the following:

 (1) (2) ...        true if 1st, 2nd, etc., capture matched
 (<NAME>) ('NAME')  true if named capture matched
 (?=...) (?<=...)   true if subpattern matches
 (?!...) (?<!...)   true if subpattern fails to match
 (?{ CODE })        true if code returns a true value
 (R)                true if evaluating inside recursion
 (R1) (R2) ...      true if directly inside capture group 1, 2, etc.
 (R&NAME)           true if directly inside named capture
 (DEFINE)           always false; for defining named subpatterns

The S<<-- HERE> shows whereabouts in the regular expression the problem was
discovered.  See L<perlre>.

=item Switch (?(condition)... not terminated in regex; marked by
S<<-- HERE> in m/%s/

(F) You omitted to close a (?(condition)...) block somewhere
in the pattern.  Add a closing parenthesis in the appropriate
position.  See L<perlre>.

=item switching effective %s is not implemented

(F) While under the C<use filetest> pragma, we cannot switch the real
and effective uids or gids.

=item syntax error

(F) Probably means you had a syntax error.  Common reasons include:

    A keyword is misspelled.
    A semicolon is missing.
    A comma is missing.
    An opening or closing parenthesis is missing.
    An opening or closing brace is missing.
    A closing quote is missing.

Often there will be another error message associated with the syntax
error giving more information.  (Sometimes it helps to turn on B<-w>.)
The error message itself often tells you where it was in the line when
it decided to give up.  Sometimes the actual error is several tokens
before this, because Perl is good at understanding random input.
Occasionally the line number may be misleading, and once in a blue moon
the only way to figure out what's triggering the error is to call
C<perl -c> repeatedly, chopping away half the program each time to see
if the error went away.  Sort of the cybernetic version of S<20 questions>.

=item syntax error at line %d: '%s' unexpected

(A) You've accidentally run your script through the Bourne shell instead
of Perl.  Check the #! line, or manually feed your script into Perl
yourself.

=item syntax error in file %s at line %d, next 2 tokens "%s"

(F) This error is likely to occur if you run a perl5 script through
a perl4 interpreter, especially if the next 2 tokens are "use strict"
or "my $var" or "our $var".

=item Syntax error in (?[...]) in regex; marked by <-- HERE in m/%s/

(F) Perl could not figure out what you meant inside this construct; this
notifies you that it is giving up trying.

=item %s syntax OK

(F) The final summary message when a C<perl -c> succeeds.

=item sysread() on closed filehandle %s

(W closed) You tried to read from a closed filehandle.

=item sysread() on unopened filehandle %s

(W unopened) You tried to read from a filehandle that was never opened.

=item System V %s is not implemented on this machine

(F) You tried to do something with a function beginning with "sem",
"shm", or "msg" but that System V IPC is not implemented in your
machine.  In some machines the functionality can exist but be
unconfigured.  Consult your system support.

=item syswrite() on closed filehandle %s

(W closed) The filehandle you're writing to got itself closed sometime
before now.  Check your control flow.

=item C<-T> and C<-B> not implemented on filehandles

(F) Perl can't peek at the stdio buffer of filehandles when it doesn't
know about your kind of stdio.  You'll have to use a filename instead.

=item Target of goto is too deeply nested

(F) You tried to use C<goto> to reach a label that was too deeply nested
for Perl to reach.  Perl is doing you a favor by refusing.

=item telldir() attempted on invalid dirhandle %s

(W io) The dirhandle you tried to telldir() is either closed or not really
a dirhandle.  Check your control flow.

=item tell() on unopened filehandle

(W unopened) You tried to use the tell() function on a filehandle that
was either never opened or has since been closed.

=item The crypt() function is unimplemented due to excessive paranoia.

(F) Configure couldn't find the crypt() function on your machine,
probably because your vendor didn't supply it, probably because they
think the U.S. Government thinks it's a secret, or at least that they
will continue to pretend that it is.  And if you quote me on that, I
will deny it.

=item The experimental declared_refs feature is not enabled

(F) To declare references to variables, as in C<my \%x>, you must first enable
the feature:

    no warnings "experimental::declared_refs";
    use feature "declared_refs";

=item The %s function is unimplemented

(F) The function indicated isn't implemented on this architecture,
according to the probings of Configure.

=item The private_use feature is experimental

(S experimental::private_use) This feature is actually a hook for future
use.

=item The stat preceding %s wasn't an lstat

(F) It makes no sense to test the current stat buffer for symbolic
linkhood if the last stat that wrote to the stat buffer already went
past the symlink to get to the real file.  Use an actual filename
instead.

=item The Unicode property wildcards feature is experimental

(S experimental::uniprop_wildcards) This feature is experimental
and its behavior may in any future release of perl.  See
L<perlunicode/Wildcards in Property Values>.

=item The 'unique' attribute may only be applied to 'our' variables

(F) This attribute was never supported on C<my> or C<sub> declarations.

=item This Perl can't reset CRTL environ elements (%s)

=item This Perl can't set CRTL environ elements (%s=%s)

(W internal) Warnings peculiar to VMS.  You tried to change or delete an
element of the CRTL's internal environ array, but your copy of Perl
wasn't built with a CRTL that contained the setenv() function.  You'll
need to rebuild Perl with a CRTL that does, or redefine
F<PERL_ENV_TABLES> (see L<perlvms>) so that the environ array isn't the
target of the change to
%ENV which produced the warning.

=item This Perl has not been built with support for randomized hash key traversal but something called Perl_hv_rand_set().

(F) Something has attempted to use an internal API call which
depends on Perl being compiled with the default support for randomized hash
key traversal, but this Perl has been compiled without it.  You should
report this warning to the relevant upstream party, or recompile perl
with default options.

=item This use of my() in false conditional is no longer allowed

(F) You used a declaration similar to C<my $x if 0>.  There
has been a long-standing bug in Perl that causes a lexical variable
not to be cleared at scope exit when its declaration includes a false
conditional.  Some people have exploited this bug to achieve a kind of
static variable.  Since we intend to fix this bug, we don't want people
relying on this behavior.  You can achieve a similar static effect by
declaring the variable in a separate block outside the function, eg

    sub f { my $x if 0; return $x++ }

becomes

    { my $x; sub f { return $x++ } }

Beginning with perl 5.10.0, you can also use C<state> variables to have
lexicals that are initialized only once (see L<feature>):

    sub f { state $x; return $x++ }

This use of C<my()> in a false conditional was deprecated beginning in
Perl 5.10 and became a fatal error in Perl 5.30.

=item Timeout waiting for another thread to define \p{%s}

(F) The first time a user-defined property
(L<perlunicode/User-Defined Character Properties>) is used, its
definition is looked up and converted into an internal form for more
efficient handling in subsequent uses.  There could be a race if two or
more threads tried to do this processing nearly simultaneously.
Instead, a critical section is created around this task, locking out all
but one thread from doing it.  This message indicates that the thread
that is doing the conversion is taking an unexpectedly long time.  The
timeout exists solely to prevent deadlock; it's long enough that the
system was likely thrashing and about to crash.  There is no real remedy but
rebooting.

=item times not implemented

(F) Your version of the C library apparently doesn't do times().  I
suspect you're not running on Unix.

=item "-T" is on the #! line, it must also be used on the command line

(X) The #! line (or local equivalent) in a Perl script contains
the B<-T> option (or the B<-t> option), but Perl was not invoked with
B<-T> in its command line.  This is an error because, by the time
Perl discovers a B<-T> in a script, it's too late to properly taint
everything from the environment.  So Perl gives up.

If the Perl script is being executed as a command using the #!
mechanism (or its local equivalent), this error can usually be
fixed by editing the #! line so that the B<-%c> option is a part of
Perl's first argument: e.g. change C<perl -n -%c> to C<perl -%c -n>.

If the Perl script is being executed as C<perl scriptname>, then the
B<-%c> option must appear on the command line: C<perl -%c scriptname>.

=item To%s: illegal mapping '%s'

(F) You tried to define a customized To-mapping for lc(), lcfirst,
uc(), or ucfirst() (or their string-inlined versions), but you
specified an illegal mapping.
See L<perlunicode/"User-Defined Character Properties">.

=item Too deeply nested ()-groups

(F) Your template contains ()-groups with a ridiculously deep nesting level.

=item Too few args to syscall

(F) There has to be at least one argument to syscall() to specify the
system call to call, silly dilly.

=item Too few arguments for subroutine '%s' (got %d; expected %d)

(F) A subroutine using a signature fewer arguments than required by the
signature.  The caller of the subroutine is presumably at fault.

The message attempts to include the name of the called subroutine.  If
the subroutine has been aliased, the subroutine's original name will be
shown, regardless of what name the caller used. It will also indicate the
number of arguments given and the number expected.

=item Too few arguments for subroutine '%s' (got %d; expected at least %d)

Similar to the previous message but for subroutines that accept a variable
number of arguments.

=item Too late for "-%s" option

(X) The #! line (or local equivalent) in a Perl script contains the
B<-M>, B<-m> or B<-C> option.

In the case of B<-M> and B<-m>, this is an error because those options
are not intended for use inside scripts.  Use the C<use> pragma instead.

The B<-C> option only works if it is specified on the command line as
well (with the same sequence of letters or numbers following).  Either
specify this option on the command line, or, if your system supports
it, make your script executable and run it directly instead of passing
it to perl.

=item Too late to run %s block

(W void) A CHECK or INIT block is being defined during run time proper,
when the opportunity to run them has already passed.  Perhaps you are
loading a file with C<require> or C<do> when you should be using C<use>
instead.  Or perhaps you should put the C<require> or C<do> inside a
BEGIN block.

=item Too many args to syscall

(F) Perl supports a maximum of only 14 args to syscall().

=item Too many arguments for %s

(F) The function requires fewer arguments than you specified.

=item Too many arguments for subroutine '%s' (got %d; expected %d)

(F) A subroutine using a signature received more arguments than permitted
by the signature.  The caller of the subroutine is presumably at fault.

The message attempts to include the name of the called subroutine. If the
subroutine has been aliased, the subroutine's original name will be shown,
regardless of what name the caller used. It will also indicate the number
of arguments given and the number expected.

=item Too many arguments for subroutine '%s' (got %d; expected at most %d)

Similar to the previous message but for subroutines that accept a variable
number of arguments.

=item Too many nested open parens in regex; marked by <-- HERE in m/%s/

(F) You have exceeded the number of open C<"("> parentheses that haven't
been matched by corresponding closing ones.  This limit prevents eating
up too much memory.  It is initially set to 1000, but may be changed by
setting C<${^RE_COMPILE_RECURSION_LIMIT}> to some other value.  This may
need to be done in a BEGIN block before the regular expression pattern
is compiled.

=item Too many nested BEGIN blocks, maximum of %d allowed

(F) You have executed code that nests too many BEGIN blocks inside of
each other, either explicitly as BEGIN{} or implicitly as use statements.
This limit defaults to a rather high number which should not be exceeded
in normal circumstances, and triggering likely indicates something is
very wrong in your code. For instance infinite recursion of eval and
BEGIN blocks is known to trigger this error.

If you know that you have good reason to exceed the limit you can change
it by setting C<${^MAX_NESTED_EVAL_BEGIN_BLOCKS}> to a different value from
the default of 1000.

=item Too many capture groups (limit is %d) in regex m/%s/

(F) You have too many capture groups in your regex pattern. You need to rework
your pattern to use less capture groups.

=item Too many )'s

(A) You've accidentally run your script through B<csh> instead of Perl.
Check the #! line, or manually feed your script into Perl yourself.

=item Too many ('s

(A) You've accidentally run your script through B<csh> instead of Perl.
Check the #! line, or manually feed your script into Perl yourself.

=item Trailing \ in regex m/%s/

(F) The regular expression ends with an unbackslashed backslash.
Backslash it.   See L<perlre>.

=item Transliteration pattern not terminated

(F) The lexer couldn't find the interior delimiter of a tr/// or tr[][]
or y/// or y[][] construct.  Missing the leading C<$> from variables
C<$tr> or C<$y> may cause this error.

=item Transliteration replacement not terminated

(F) The lexer couldn't find the final delimiter of a tr///, tr[][],
y/// or y[][] construct.

=item Treating %s::INIT block as BEGIN block as workaround

(S) A package is using an old version of C<Module::Install::DSL> to
install, which makes unsafe assumptions about when INIT blocks will be
called. Because C<Module::Install::DSL> is used to install other modules
and is difficult to upgrade we have a special workaround in place to
deal with this. Unless you are a maintainer of an affected module you
can ignore this warning. We emit it only as a sanity check.

=item '%s' trapped by operation mask

(F) You tried to use an operator from a Safe compartment in which it's
disallowed.  See L<Safe>.

=item truncate not implemented

(F) Your machine doesn't implement a file truncation mechanism that
Configure knows about.

=item try/catch is experimental

(S experimental::try) This warning is emitted if you use the C<try> and
C<catch> syntax. This syntax is currently experimental and its behaviour may
change in future releases of Perl.

=item try/catch/finally is experimental

(S experimental::try) This warning is emitted if you use the C<try> and
C<catch> syntax with a C<finally> block. This syntax is currently experimental
and its behaviour may change in future releases of Perl.

=item Type of arg %d to &CORE::%s must be %s

(F) The subroutine in question in the CORE package requires its argument
to be a hard reference to data of the specified type.  Overloading is
ignored, so a reference to an object that is not the specified type, but
nonetheless has overloading to handle it, will still not be accepted.

=item Type of arg %d to %s must be %s (not %s)

(F) This function requires the argument in that position to be of a
certain type.  Arrays must be @NAME or C<@{EXPR}>.  Hashes must be
%NAME or C<%{EXPR}>.  No implicit dereferencing is allowed--use the
{EXPR} forms as an explicit dereference.  See L<perlref>.

=item umask not implemented

(F) Your machine doesn't implement the umask function and you tried to
use it to restrict permissions for yourself (EXPR & 0700).

=item Unbalanced context: %d more PUSHes than POPs

(S internal) The exit code detected an internal inconsistency in how
many execution contexts were entered and left.

=item Unbalanced saves: %d more saves than restores

(S internal) The exit code detected an internal inconsistency in how
many values were temporarily localized.

=item Unbalanced scopes: %d more ENTERs than LEAVEs

(S internal) The exit code detected an internal inconsistency in how
many blocks were entered and left.

=item Unbalanced string table refcount: (%d) for "%s"

(S internal) On exit, Perl found some strings remaining in the shared
string table used for copy on write and for hash keys.  The entries
should have been freed, so this indicates a bug somewhere.

=item Unbalanced tmps: %d more allocs than frees

(S internal) The exit code detected an internal inconsistency in how
many mortal scalars were allocated and freed.

=item Undefined format "%s" called

(F) The format indicated doesn't seem to exist.  Perhaps it's really in
another package?  See L<perlform>.

=item Undefined sort subroutine "%s" called

(F) The sort comparison routine specified doesn't seem to exist.
Perhaps it's in a different package?  See L<perlfunc/sort>.

=item Undefined subroutine &%s called

(F) The subroutine indicated hasn't been defined, or if it was, it has
since been undefined.

=item Undefined subroutine called

(F) The anonymous subroutine you're trying to call hasn't been defined,
or if it was, it has since been undefined.

=item Undefined subroutine in sort

(F) The sort comparison routine specified is declared but doesn't seem
to have been defined yet.  See L<perlfunc/sort>.

=item Undefined top format "%s" called

(F) The format indicated doesn't seem to exist.  Perhaps it's really in
another package?  See L<perlform>.

=item Undefined value assigned to typeglob

(W misc) An undefined value was assigned to a typeglob, a la
C<*foo = undef>.  This does nothing.  It's possible that you really mean
C<undef *foo>.

=item %s: Undefined variable

(A) You've accidentally run your script through B<csh> instead of Perl.
Check the #! line, or manually feed your script into Perl yourself.

=item Unescaped left brace in regex is illegal here in regex;
marked by S<<-- HERE> in m/%s/

(F) The simple rule to remember, if you want to
match a literal C<"{"> character (U+007B C<LEFT CURLY BRACKET>) in a
regular expression pattern, is to escape each literal instance of it in
some way.  Generally easiest is to precede it with a backslash, like
C<"\{"> or enclose it in square brackets (C<"[{]">).  If the pattern
delimiters are also braces, any matching right brace (C<"}">) should
also be escaped to avoid confusing the parser, for example,

 qr{abc\{def\}ghi}

Forcing literal C<"{"> characters to be escaped enables the Perl
language to be extended in various ways in future releases.  To avoid
needlessly breaking existing code, the restriction is not enforced in
contexts where there are unlikely to ever be extensions that could
conflict with the use there of C<"{"> as a literal.  Those that are
not potentially ambiguous do not warn; those that are do raise a
non-deprecation warning.

The contexts where no warnings or errors are raised are:

=over 4

=item *

as the first character in a pattern, or following C<"^"> indicating to
anchor the match to the beginning of a line.

=item *

as the first character following a C<"|"> indicating alternation.

=item *

as the first character in a parenthesized grouping like

 /foo({bar)/
 /foo(?:{bar)/

=item *

as the first character following a quantifier

 /\s*{/

=back

=for comment
The text of the message above is mostly duplicated below (with changes)
to allow splain (and 'use diagnostics') to work.  Since one is fatal,
and one not, they can't be combined as one message.  Perhaps perldiag
could be enhanced to handle this case.

=item Unescaped left brace in regex is passed through in regex; marked by S<<-- HERE> in m/%s/

(W regexp)  The simple rule to remember, if you want to
match a literal C<"{"> character (U+007B C<LEFT CURLY BRACKET>) in a
regular expression pattern, is to escape each literal instance of it in
some way.  Generally easiest is to precede it with a backslash, like
C<"\{"> or enclose it in square brackets (C<"[{]">).  If the pattern
delimiters are also braces, any matching right brace (C<"}">) should
also be escaped to avoid confusing the parser, for example,

 qr{abc\{def\}ghi}

Forcing literal C<"{"> characters to be escaped enables the Perl
language to be extended in various ways in future releases.  To avoid
needlessly breaking existing code, the restriction is not enforced in
contexts where there are unlikely to ever be extensions that could
conflict with the use there of C<"{"> as a literal.  Those that are
not potentially ambiguous do not warn; those that are raise this
warning.  This makes sure that an inadvertent typo doesn't silently
cause the pattern to compile to something unintended.

The contexts where no warnings or errors are raised are:

=over 4

=item *

as the first character in a pattern, or following C<"^"> indicating to
anchor the match to the beginning of a line.

=item *

as the first character following a C<"|"> indicating alternation.

=item *

as the first character in a parenthesized grouping like

 /foo({bar)/
 /foo(?:{bar)/

=item *

as the first character following a quantifier

 /\s*{/

=back

=item Unescaped literal '%c' in regex; marked by <-- HERE in m/%s/

(W regexp) (only under C<S<use re 'strict'>>)

Within the scope of C<S<use re 'strict'>> in a regular expression
pattern, you included an unescaped C<}> or C<]> which was interpreted
literally.  These two characters are sometimes metacharacters, and
sometimes literals, depending on what precedes them in the
pattern.  This is unlike the similar C<)> which is always a
metacharacter unless escaped.

This action at a distance, perhaps a large distance, can lead to Perl
silently misinterpreting what you meant, so when you specify that you
want extra checking by C<S<use re 'strict'>>, this warning is generated.
If you meant the character as a literal, simply confirm that to Perl by
preceding the character with a backslash, or make it into a bracketed
character class (like C<[}]>).  If you meant it as closing a
corresponding C<[> or C<{>, you'll need to look back through the pattern
to find out why that isn't happening.

=item unexec of %s into %s failed!

(F) The unexec() routine failed for some reason.  See your local FSF
representative, who probably put it there in the first place.

=item Unexpected binary operator '%c' with no preceding operand in regex;
marked by S<<-- HERE> in m/%s/

(F) You had something like this:

 (?[ | \p{Digit} ])

where the C<"|"> is a binary operator with an operand on the right, but
no operand on the left.

=item Unexpected character in regex; marked by S<<-- HERE> in m/%s/

(F) You had something like this:

 (?[ z ])

Within C<(?[ ])>, no literal characters are allowed unless they are
within an inner pair of square brackets, like

 (?[ [ z ] ])

Another possibility is that you forgot a backslash.  Perl isn't smart
enough to figure out what you really meant.

=item Unexpected characters while parsing class :isa attribute: %s

(F) You tried to specify something other than a single class name with an
optional trailing verison number as the value for a C<class> C<:isa>
attribute.  This confused the parser.

=item Unexpected exit %u

(S) exit() was called or the script otherwise finished gracefully when
C<PERL_EXIT_WARN> was set in C<PL_exit_flags>.

=item Unexpected exit failure %d

(S) An uncaught die() was called when C<PERL_EXIT_WARN> was set in
C<PL_exit_flags>.

=item Unexpected ')' in regex; marked by S<<-- HERE> in m/%s/

(F) You had something like this:

 (?[ ( \p{Digit} + ) ])

The C<")"> is out-of-place.  Something apparently was supposed to
be combined with the digits, or the C<"+"> shouldn't be there, or
something like that.  Perl can't figure out what was intended.

=item Unexpected ']' with no following ')' in (?[... in regex; marked by
<-- HERE in m/%s/

(F) While parsing an extended character class a ']' character was
encountered at a point in the definition where the only legal use of
']' is to close the character class definition as part of a '])', you
may have forgotten the close paren, or otherwise confused the parser.

=item Unexpected '(' with no preceding operator in regex; marked by
S<<-- HERE> in m/%s/

(F) You had something like this:

 (?[ \p{Digit} ( \p{Lao} + \p{Thai} ) ])

There should be an operator before the C<"(">, as there's
no indication as to how the digits are to be combined
with the characters in the Lao and Thai scripts.

=item Unicode non-character U+%X is not recommended for open interchange

(S nonchar) Certain codepoints, such as U+FFFE and U+FFFF, are
defined by the Unicode standard to be non-characters.  Those
are legal codepoints, but are reserved for internal use; so,
applications shouldn't attempt to exchange them.  An application
may not be expecting any of these characters at all, and receiving
them may lead to bugs.  If you know what you are doing you can
turn off this warning by C<no warnings 'nonchar';>.

This is not really a "severe" error, but it is supposed to be
raised by default even if warnings are not enabled, and currently
the only way to do that in Perl is to mark it as serious.

=item Unicode property wildcard not terminated

(F) A Unicode property wildcard looks like a delimited regular
expression pattern (all within the braces of the enclosing C<\p{...}>.
The closing delimtter to match the opening one was not found.  If the
opening one is escaped by preceding it with a backslash, the closing one
must also be so escaped.

=item Unicode string properties are not implemented in (?[...]) in
regex; marked by <-- HERE in m/%s/

(F) A Unicode string property is one which expands to a sequence of
multiple characters.  An example is C<\p{name=KATAKANA LETTER AINU P}>,
which is comprised of the sequence C<\N{KATAKANA LETTER SMALL H}>
followed by C<\N{COMBINING KATAKANA-HIRAGANA SEMI-VOICED SOUND MARK}>.
Extended character classes, C<(?[...])> currently cannot handle these.

=item Unicode surrogate U+%X is illegal in UTF-8

(S surrogate) You had a UTF-16 surrogate in a context where they are
not considered acceptable.  These code points, between U+D800 and
U+DFFF (inclusive), are used by Unicode only for UTF-16.  However, Perl
internally allows all unsigned integer code points (up to the size limit
available on your platform), including surrogates.  But these can cause
problems when being input or output, which is likely where this message
came from.  If you really really know what you are doing you can turn
off this warning by C<no warnings 'surrogate';>.

=item Unimplemented

(F) In Perl 5.12 and above, you have executed an
L<ellipsis statement|perlsyn/"The Ellipsis Statement">.  This is a
bare C<...;>, meant to be used to allow you to outline code that
is to be written, but is not complete, similar to the following:

    sub not_done_yet {
      my($self, $arg1, $arg2) = @_;
      ...
    }

If C<not_done_yet()> is called, Perl will die with an C<Unimplemented> error
at the line containing C<...>.

=item Unknown charname '%s'

(F) The name you used inside C<\N{}> is unknown to Perl.  Check the
spelling.  You can say C<use charnames ":loose"> to not have to be
so precise about spaces, hyphens, and capitalization on standard Unicode
names.  (Any custom aliases that have been created must be specified
exactly, regardless of whether C<:loose> is used or not.)  This error may
also happen if the C<\N{}> is not in the scope of the corresponding
C<S<use charnames>>.

=item Unknown '(*...)' construct '%s' in regex; marked by <-- HERE in m/%s/

(F) The C<(*> was followed by something that the regular expression
compiler does not recognize.  Check your spelling.

=item Unknown error

(P) Perl was about to print an error message in C<$@>, but the C<$@> variable
did not exist, even after an attempt to create it.

=item Unknown locale category %d

=item Unknown locale category %d; can't set it to %s

(W locale) You used a locale category that perl doesn't recognize, so it
cannot carry out your request.  Check that you are using a valid
category.  If so, see L<perllocale/Multi-threaded> for advice on
reporting this as a bug, and for modifying perl locally to accommodate
your needs.

=item Unknown open() mode '%s'

(F) The second argument of 3-argument open() is not among the list
of valid modes: C<< < >>, C<< > >>, C<<< >> >>>, C<< +< >>,
C<< +> >>, C<<< +>> >>>, C<-|>, C<|->, C<< <& >>, C<< >& >>.

=item Unknown PerlIO layer "%s"

(W layer) An attempt was made to push an unknown layer onto the Perl I/O
system.  (Layers take care of transforming data between external and
internal representations.)  Note that some layers, such as C<mmap>,
are not supported in all environments.  If your program didn't
explicitly request the failing operation, it may be the result of the
value of the environment variable PERLIO.

=item Unknown process %x sent message to prime_env_iter: %s

(P) An error peculiar to VMS.  Perl was reading values for %ENV before
iterating over it, and someone else stuck a message in the stream of
data Perl expected.  Someone's very confused, or perhaps trying to
subvert Perl's population of %ENV for nefarious purposes.

=item Unknown regexp modifier "/%s"

(F) Alphanumerics immediately following the closing delimiter
of a regular expression pattern are interpreted by Perl as modifier
flags for the regex.  One of the ones you specified is invalid.  One way
this can happen is if you didn't put in white space between the end of
the regex and a following alphanumeric operator:

 if ($a =~ /foo/and $bar == 3) { ... }

The C<"a"> is a valid modifier flag, but the C<"n"> is not, and raises
this error.  Likely what was meant instead was:

 if ($a =~ /foo/ and $bar == 3) { ... }

=item Unknown "re" subpragma '%s' (known ones are: %s)

(W) You tried to use an unknown subpragma of the "re" pragma.

=item Unknown switch condition (?(...)) in regex; marked by S<<-- HERE> in
m/%s/

(F) The condition part of a (?(condition)if-clause|else-clause) construct
is not known.  The condition must be one of the following:

 (1) (2) ...            true if 1st, 2nd, etc., capture matched
 (<NAME>) ('NAME')      true if named capture matched
 (?=...) (?<=...)       true if subpattern matches
 (*pla:...) (*plb:...)  true if subpattern matches; also
                             (*positive_lookahead:...)
                             (*positive_lookbehind:...)
 (*nla:...) (*nlb:...)  true if subpattern fails to match; also
                             (*negative_lookahead:...)
                             (*negative_lookbehind:...)
 (?{ CODE })            true if code returns a true value
 (R)                    true if evaluating inside recursion
 (R1) (R2) ...          true if directly inside capture group 1, 2,
                             etc.
 (R&NAME)               true if directly inside named capture
 (DEFINE)               always false; for defining named subpatterns

The S<<-- HERE> shows whereabouts in the regular expression the problem was
discovered.  See L<perlre>.

=item Unknown Unicode option letter '%c'

(F) You specified an unknown Unicode option.  See
L<perlrun|perlrun/-C [numberE<sol>list]> documentation of the C<-C> switch
for the list of known options.

=item Unknown Unicode option value %d

(F) You specified an unknown Unicode option.  See
L<perlrun|perlrun/-C [numberE<sol>list]> documentation of the C<-C> switch
for the list of known options.

=item Unknown user-defined property name \p{%s}

(F) You specified to use a property within the C<\p{...}> which was a
syntactically valid user-defined property, but no definition was found
for it by the time one was required to proceed.  Check your spelling.
See L<perlunicode/User-Defined Character Properties>.

=item Unknown verb pattern '%s' in regex; marked by S<<-- HERE> in m/%s/

(F) You either made a typo or have incorrectly put a C<*> quantifier
after an open brace in your pattern.  Check the pattern and review
L<perlre> for details on legal verb patterns.

=item Unknown warnings category '%s'

(F) An error issued by the C<warnings> pragma.  You specified a warnings
category that is unknown to perl at this point.

Note that if you want to enable a warnings category registered by a
module (e.g. C<use warnings 'File::Find'>), you must have loaded this
module first.

=item Unmatched [ in regex; marked by S<<-- HERE> in m/%s/

(F) The brackets around a character class must match.  If you wish to
include a closing bracket in a character class, backslash it or put it
first.  The S<<-- HERE> shows whereabouts in the regular expression the
problem was discovered.  See L<perlre>.

=item Unmatched ( in regex; marked by S<<-- HERE> in m/%s/

=item Unmatched ) in regex; marked by S<<-- HERE> in m/%s/

(F) Unbackslashed parentheses must always be balanced in regular
expressions.  If you're a vi user, the % key is valuable for finding
the matching parenthesis.  The S<<-- HERE> shows whereabouts in the
regular expression the problem was discovered.  See L<perlre>.

=item Unmatched right %s bracket

(F) The lexer counted more closing curly or square brackets than opening
ones, so you're probably missing a matching opening bracket.  As a
general rule, you'll find the missing one (so to speak) near the place
you were last editing.

=item Unquoted string "%s" may clash with future reserved word

(W reserved) You used a bareword that might someday be claimed as a
reserved word.  It's best to put such a word in quotes, or capitalize it
somehow, or insert an underbar into it.  You might also declare it as a
subroutine.

=item Unrecognized character %s; marked by S<<-- HERE> after %s near column
%d

(F) The Perl parser has no idea what to do with the specified character
in your Perl script (or eval) near the specified column.  Perhaps you
tried  to run a compressed script, a binary program, or a directory as
a Perl program.

=item Unrecognized class attribute %s

(F) You attempted to add a named attribute to a C<class> definition, but
perl does not recognise the name of the requested attribute.

=item Unrecognized escape \%c in character class in regex; marked by
S<<-- HERE> in m/%s/

(F) You used a backslash-character combination which is not
recognized by Perl inside character classes.  This is a fatal
error when the character class is used within C<(?[ ])>.

=item Unrecognized escape \%c in character class passed through in regex; 
marked by S<<-- HERE> in m/%s/

(W regexp) You used a backslash-character combination which is not
recognized by Perl inside character classes.  The character was
understood literally, but this may change in a future version of Perl.
The S<<-- HERE> shows whereabouts in the regular expression the
escape was discovered.

=item Unrecognized escape \%c passed through

(W misc) You used a backslash-character combination which is not
recognized by Perl.  The character was understood literally, but this may
change in a future version of Perl.

=item Unrecognized escape \%s passed through in regex; marked by
S<<-- HERE> in m/%s/

(W regexp) You used a backslash-character combination which is not
recognized by Perl.  The character(s) were understood literally, but
this may change in a future version of Perl.  The S<<-- HERE> shows
whereabouts in the regular expression the escape was discovered.

=item Unrecognized field attribute %s

(F) You attempted to add a named attribute to a C<field> definition, but
perl does not recognise the name of the requested attribute.

=item Unrecognized signal name "%s"

(F) You specified a signal name to the kill() function that was not
recognized.  Say C<kill -l> in your shell to see the valid signal names
on your system.

=item Unrecognized switch: -%s  (-h will show valid options)

(F) You specified an illegal option to Perl.  Don't do that.  (If you
think you didn't do that, check the #! line to see if it's supplying the
bad switch on your behalf.)

=item Unsuccessful %s on filename containing newline

(W newline) A file operation was attempted on a filename, and that
operation failed, PROBABLY because the filename contained a newline,
PROBABLY because you forgot to chomp() it off.  See L<perlfunc/chomp>.

=item Unsupported directory function "%s" called

(F) Your machine doesn't support opendir() and readdir().

=item Unsupported function %s

(F) This machine doesn't implement the indicated function, apparently.
At least, Configure doesn't think so.

=item Unsupported function fork

(F) Your version of executable does not support forking.

Note that under some systems, like OS/2, there may be different flavors
of Perl executables, some of which may support fork, some not.  Try
changing the name you call Perl by to C<perl_>, C<perl__>, and so on.

=item Unsupported script encoding %s

(F) Your program file begins with a Unicode Byte Order Mark (BOM) which
declares it to be in a Unicode encoding that Perl cannot read.

=item Unsupported socket function "%s" called

(F) Your machine doesn't support the Berkeley socket mechanism, or at
least that's what Configure thought.

=item Unterminated '(*...' argument in regex; marked by <-- HERE in m/%s/

(F) You used a pattern of the form C<(*...:...)> but did not terminate
the pattern with a C<)>.  Fix the pattern and retry.

=item Unterminated attribute list

(F) The lexer found something other than a simple identifier at the
start of an attribute, and it wasn't a semicolon or the start of a
block.  Perhaps you terminated the parameter list of the previous
attribute too soon.  See L<attributes>.

=item Unterminated attribute parameter in attribute list

(F) The lexer saw an opening (left) parenthesis character while parsing
an attribute list, but the matching closing (right) parenthesis
character was not found.  You may need to add (or remove) a backslash
character to get your parentheses to balance.  See L<attributes>.

=item Unterminated compressed integer

(F) An argument to unpack("w",...) was incompatible with the BER
compressed integer format and could not be converted to an integer.
See L<perlfunc/pack>.

=item Unterminated '(*...' construct in regex; marked by <-- HERE in m/%s/

(F) You used a pattern of the form C<(*...)> but did not terminate
the pattern with a C<)>.  Fix the pattern and retry.

=item Unterminated delimiter for here document

(F) This message occurs when a here document label has an initial
quotation mark but the final quotation mark is missing.  Perhaps
you wrote:

    <<"foo

instead of:

    <<"foo"

=item Unterminated \g... pattern in regex; marked by S<<-- HERE> in m/%s/

=item Unterminated \g{...} pattern in regex; marked by S<<-- HERE> in m/%s/

(F) In a regular expression, you had a C<\g> that wasn't followed by a
proper group reference.  In the case of C<\g{>, the closing brace is
missing; otherwise the C<\g> must be followed by an integer.  Fix the
pattern and retry.

=item Unterminated <> operator

(F) The lexer saw a left angle bracket in a place where it was expecting
a term, so it's looking for the corresponding right angle bracket, and
not finding it.  Chances are you left some needed parentheses out
earlier in the line, and you really meant a "less than".

=item Unterminated verb pattern argument in regex; marked by S<<-- HERE> in
m/%s/

(F) You used a pattern of the form C<(*VERB:ARG)> but did not terminate
the pattern with a C<)>.  Fix the pattern and retry.

=item Unterminated verb pattern in regex; marked by S<<-- HERE> in m/%s/

(F) You used a pattern of the form C<(*VERB)> but did not terminate
the pattern with a C<)>.  Fix the pattern and retry.

=item untie attempted while %d inner references still exist

(W untie) A copy of the object returned from C<tie> (or C<tied>) was
still valid when C<untie> was called.

=item Usage: POSIX::%s(%s)

(F) You called a POSIX function with incorrect arguments.
See L<POSIX/FUNCTIONS> for more information.

=item Usage: Win32::%s(%s)

(F) You called a Win32 function with incorrect arguments.
See L<Win32> for more information.

=item $[ used in %s (did you mean $] ?)

(W syntax) You used C<$[> in a comparison, such as:

    if ($[ > 5.006) {
	...
    }

You probably meant to use C<$]> instead.  C<$[> is the base for indexing
arrays.  C<$]> is the Perl version number in decimal.

=item Use "%s" instead of "%s"

(F) The second listed construct is no longer legal.  Use the first one
instead.

=item Useless assignment to a temporary

(W misc) You assigned to an lvalue subroutine, but what
the subroutine returned was a temporary scalar about to
be discarded, so the assignment had no effect.

=item Useless (?-%s) - don't use /%s modifier in regex; marked by
S<<-- HERE> in m/%s/

(W regexp) You have used an internal modifier such as (?-o) that has no
meaning unless removed from the entire regexp:

    if ($string =~ /(?-o)$pattern/o) { ... }

must be written as

    if ($string =~ /$pattern/) { ... }

The S<<-- HERE> shows whereabouts in the regular expression the problem was
discovered.  See L<perlre>.

=item Useless localization of %s

(W syntax) The localization of lvalues such as C<local($x=10)> is legal,
but in fact the local() currently has no effect.  This may change at
some point in the future, but in the meantime such code is discouraged.

=item Useless (?%s) - use /%s modifier in regex; marked by S<<-- HERE> in
m/%s/

(W regexp) You have used an internal modifier such as (?o) that has no
meaning unless applied to the entire regexp:

    if ($string =~ /(?o)$pattern/) { ... }

must be written as

    if ($string =~ /$pattern/o) { ... }

The S<<-- HERE> shows whereabouts in the regular expression the problem was
discovered.  See L<perlre>.

=item Useless use of attribute "const"

(W misc) The C<const> attribute has no effect except
on anonymous closure prototypes.  You applied it to
a subroutine via L<attributes.pm|attributes>.  This is only useful
inside an attribute handler for an anonymous subroutine.

=item Useless use of /d modifier in transliteration operator

(W misc) You have used the /d modifier where the searchlist has the
same length as the replacelist.  See L<perlop> for more information
about the /d modifier.

=item Useless use of \E

(W misc) You have a \E in a double-quotish string without a C<\U>,
C<\L> or C<\Q> preceding it.

=item Useless use of greediness modifier '%c' in regex; marked by S<<-- HERE> in m/%s/

(W regexp) You specified something like these:

 qr/a{3}?/
 qr/b{1,1}+/

The C<"?"> and C<"+"> don't have any effect, as they modify whether to
match more or fewer when there is a choice, and by specifying to match
exactly a given number, there is no room left for a choice.

=item Useless use of %s in scalar context

(W scalar) You did something whose only interesting return value is a
list without a side effect in scalar context, which does not accept a
list.

For example

    my $x = sort @y;

This is not very useful, and perl currently optimizes this away.

=item Useless use of %s in void context

(W void) You did something without a side effect in a context that does
nothing with the return value, such as a statement that doesn't return a
value from a block, or the left side of a scalar comma operator.  Very
often this points not to stupidity on your part, but a failure of Perl
to parse your program the way you thought it would.  For example, you'd
get this if you mixed up your C precedence with Python precedence and
said

    $one, $two = 1, 2;

when you meant to say

    ($one, $two) = (1, 2);

Another common error is to use ordinary parentheses to construct a list
reference when you should be using square or curly brackets, for
example, if you say

    $array = (1,2);

when you should have said

    $array = [1,2];

The square brackets explicitly turn a list value into a scalar value,
while parentheses do not.  So when a parenthesized list is evaluated in
a scalar context, the comma is treated like C's comma operator, which
throws away the left argument, which is not what you want.  See
L<perlref> for more on this.

This warning will not be issued for numerical constants equal to 0 or 1
since they are often used in statements like

    1 while sub_with_side_effects();

String constants that would normally evaluate to 0 or 1 are warned
about.

=item Useless use of (?-p) in regex; marked by S<<-- HERE> in m/%s/

(W regexp) The C<p> modifier cannot be turned off once set.  Trying to do
so is futile.

=item Useless use of "re" pragma

(W) You did C<use re;> without any arguments.  That isn't very useful.

=item Useless use of %s with no values

(W syntax) You used the push() or unshift() function with no arguments
apart from the array, like C<push(@x)> or C<unshift(@foo)>.  That won't
usually have any effect on the array, so is completely useless.  It's
possible in principle that push(@tied_array) could have some effect
if the array is tied to a class which implements a PUSH method.  If so,
you can write it as C<push(@tied_array,())> to avoid this warning.

=item "use" not allowed in expression

(F) The "use" keyword is recognized and executed at compile time, and
returns no useful value.  See L<perlmod>.

=item Use of @_ in %s with signatured subroutine is experimental

(S experimental::args_array_with_signatures) An expression involving the
C<@_> arguments array was found in a subroutine that uses a signature.
This is experimental because the interaction between the arguments
array and parameter handling via signatures is not guaranteed to remain
stable in any future version of Perl, and such code should be avoided.

=item Use of bare << to mean <<"" is forbidden

(F) You are now required to use the explicitly quoted form if you wish
to use an empty line as the terminator of the here-document.

Use of a bare terminator was deprecated in Perl 5.000, and is a fatal
error as of Perl 5.28.

=item Use of /c modifier is meaningless in s///

(W regexp) You used the /c modifier in a substitution.  The /c
modifier is not presently meaningful in substitutions.

=item Use of /c modifier is meaningless without /g

(W regexp) You used the /c modifier with a regex operand, but didn't
use the /g modifier.  Currently, /c is meaningful only when /g is
used.  (This may change in the future.)

=item Use of code point 0x%s is not allowed; the permissible max is 0x%X

=item Use of code point 0x%s is not allowed; the permissible max is 0x%X
in regex; marked by <-- HERE in m/%s/

(F) You used a code point that is not allowed, because it is too large.
Unicode only allows code points up to 0x10FFFF, but Perl allows much
larger ones. Earlier versions of Perl allowed code points above IV_MAX
(0x7FFFFFF on 32-bit platforms, 0x7FFFFFFFFFFFFFFF on 64-bit platforms),
however, this could possibly break the perl interpreter in some constructs,
including causing it to hang in a few cases.

If your code is to run on various platforms, keep in mind that the upper
limit depends on the platform.  It is much larger on 64-bit word sizes
than 32-bit ones.

The use of out of range code points was deprecated in Perl 5.24, and
became a fatal error in Perl 5.28.

=item Use of each() on hash after insertion without resetting hash iterator results in undefined behavior

(S internal) The behavior of C<each()> after insertion is undefined;
it may skip items, or visit items more than once.  Consider using
C<keys()> instead of C<each()>.

=item Use of := for an empty attribute list is not allowed

(F) The construction C<my $x := 42> used to parse as equivalent to
C<my $x : = 42> (applying an empty attribute list to C<$x>).
This construct was deprecated in 5.12.0, and has now been made a syntax
error, so C<:=> can be reclaimed as a new operator in the future.

If you need an empty attribute list, for example in a code generator, add
a space before the C<=>.

=item Use of %s for non-UTF-8 locale is wrong.  Assuming a UTF-8 locale

(W locale)  You are matching a regular expression using locale rules,
and the specified construct was encountered.  This construct is only
valid for UTF-8 locales, which the current locale isn't.  This doesn't
make sense.  Perl will continue, assuming a Unicode (UTF-8) locale, but
the results are likely to be wrong.

=item Use of freed value in iteration

(F) Perhaps you modified the iterated array within the loop?
This error is typically caused by code like the following:

    @a = (3,4);
    @a = () for (1,2,@a);

You are not supposed to modify arrays while they are being iterated over.
For speed and efficiency reasons, Perl internally does not do full
reference-counting of iterated items, hence deleting such an item in the
middle of an iteration causes Perl to see a freed value.

=item Use of /g modifier is meaningless in split

(W regexp) You used the /g modifier on the pattern for a C<split>
operator.  Since C<split> always tries to match the pattern
repeatedly, the C</g> has no effect.

=item Use of "goto" to jump into a construct is deprecated

(D deprecated::goto_construct) Using C<goto> to jump from an outer scope into an inner
scope is deprecated and should be avoided.

This was deprecated in Perl 5.12.

=item Use of '%s' in \p{} or \P{} is deprecated because: %s

(D deprecated::unicode_property_name) Certain properties are deprecated
by Unicode, and may eventually be removed from the Standard, at which
time Perl will follow along. In the meantime, this message is raised to
notify you.

=item Use of inherited AUTOLOAD for non-method %s::%s() is no longer allowed

(F) As an accidental feature, C<AUTOLOAD> subroutines were looked up as
methods (using the C<@ISA> hierarchy), even when the subroutines to be
autoloaded were called as plain functions (e.g. C<Foo::bar()>), not as
methods (e.g. C<< Foo->bar() >> or C<< $obj->bar() >>).

This was deprecated in Perl 5.004, and was made fatal in Perl 5.28.

=item Use of %s in printf format not supported

(F) You attempted to use a feature of printf that is accessible from
only C.  This usually means there's a better way to do it in Perl.

=item Use of '%s' is deprecated as a string delimiter

(D deprecated::delimiter_will_be_paired) You used the given character as
a starting delimiter of a string outside the scope of
S<C<use feature 'extra_paired_delimiters'>>. This character is the
mirror image of another Unicode character; within the scope of that
feature, the two are considered a pair for delimitting strings. It is
planned to make that feature the default, at which point this usage
would become illegal; hence this warning.

For now, you may live with this warning, or turn it off, but this code
will no longer compile in a future version of Perl.  Or you can turn on
S<C<use feature 'extra_paired_delimiters'>> and use the character that
is the mirror image of this one for the closing string delimiter.

=item Use of '%s' is experimental as a string delimiter

(S experimental::extra_paired_delimiters)   This warning is emitted if
you use as a string delimiter one of the non-ASCII mirror image ones
enabled by S<C<use feature 'extra_paired_delimiters'>>.  Simply suppress
the warning if you want to use the feature, but know that in doing so
you are taking the risk of using an experimental feature which may
change or be removed in a future Perl version:

=item Use of %s is not allowed in Unicode property wildcard
subpatterns in regex; marked by S<<-- HERE> in m/%s/

(F) You were using a wildcard subpattern a Unicode property value, and
the subpattern contained something that is illegal.  Not all regular
expression capabilities are legal in such subpatterns, and this is one.
Rewrite your subppattern to not use the offending construct.
See L<perlunicode/Wildcards in Property Values>.

=item Use of -l on filehandle%s

(W io) A filehandle represents an opened file, and when you opened the file
it already went past any symlink you are presumably trying to look for.
The operation returned C<undef>.  Use a filename instead.

=item Use of reference "%s" as array index

(W misc) You tried to use a reference as an array index; this probably
isn't what you mean, because references in numerical context tend
to be huge numbers, and so usually indicates programmer error.

If you really do mean it, explicitly numify your reference, like so:
C<$array[0+$ref]>.  This warning is not given for overloaded objects,
however, because you can overload the numification and stringification
operators and then you presumably know what you are doing.

=item Use of strings with code points over 0xFF as arguments to %s
operator is not allowed

(F) You tried to use one of the string bitwise operators (C<&> or C<|> or C<^> or
C<~>) on a string containing a code point over 0xFF.  The string bitwise
operators treat their operands as strings of bytes, and values beyond
0xFF are nonsensical in this context.

Certain instances became fatal in Perl 5.28; others in perl 5.32.

=item Use of strings with code points over 0xFF as arguments to vec is forbidden

(F) You tried to use L<C<vec>|perlfunc/vec EXPR,OFFSET,BITS>
on a string containing a code point over 0xFF, which is nonsensical here.

This became fatal in Perl 5.32.

=item Use of tainted arguments in %s is deprecated

(W taint, deprecated) You have supplied C<system()> or C<exec()> with multiple
arguments and at least one of them is tainted.  This used to be allowed
but will become a fatal error in a future version of perl.  Untaint your
arguments.  See L<perlsec>.

=item Use of unassigned code point or non-standalone grapheme for a
delimiter is not allowed

(F)
A grapheme is what appears to a native-speaker of a language to be a
character.  In Unicode (and hence Perl) a grapheme may actually be
several adjacent characters that together form a complete grapheme.  For
example, there can be a base character, like "R" and an accent, like a
circumflex "^", that appear when displayed to be a single character with
the circumflex hovering over the "R".  Perl currently allows things like
that circumflex to be delimiters of strings, patterns, I<etc>.  When
displayed, the circumflex would look like it belongs to the character
just to the left of it.  In order to move the language to be able to
accept graphemes as delimiters, we cannot allow the use of
delimiters which aren't graphemes by themselves.  Also, a delimiter must
already be assigned (or known to be never going to be assigned) to try
to future-proof code, for otherwise code that works today would fail to
compile if the currently unassigned delimiter ends up being something
that isn't a stand-alone grapheme.  Because Unicode is never going to
assign
L<non-character code points|perlunicode/Noncharacter code points>, nor
L<code points that are above the legal Unicode maximum|
perlunicode/Beyond Unicode code points>, those can be delimiters, and
their use is legal.

=item Use of uninitialized value%s

(W uninitialized) An undefined value was used as if it were already
defined.  It was interpreted as a "" or a 0, but maybe it was a mistake.
To suppress this warning assign a defined value to your variables.

To help you figure out what was undefined, perl will try to tell you
the name of the variable (if any) that was undefined.  In some cases
it cannot do this, so it also tells you what operation you used the
undefined value in.  Note, however, that perl optimizes your program
and the operation displayed in the warning may not necessarily appear
literally in your program.  For example, C<"that $foo"> is usually
optimized into C<"that " . $foo>, and the warning will refer to the
C<concatenation (.)> operator, even though there is no C<.> in
your program.

=item "use re 'strict'" is experimental

(S experimental::re_strict) The things that are different when a regular
expression pattern is compiled under C<'strict'> are subject to change
in future Perl releases in incompatible ways.  This means that a pattern
that compiles today may not in a future Perl release.  This warning is
to alert you to that risk.

=item Use \x{...} for more than two hex characters in regex; marked by
S<<-- HERE> in m/%s/

(F) In a regular expression, you said something like

 (?[ [ \xBEEF ] ])

Perl isn't sure if you meant this

 (?[ [ \x{BEEF} ] ])

or if you meant this

 (?[ [ \x{BE} E F ] ])

You need to add either braces or blanks to disambiguate.

=item Using just the first character returned by \N{} in character class in 
regex; marked by S<<-- HERE> in m/%s/

(W regexp) Named Unicode character escapes C<(\N{...})> may return
a multi-character sequence.  Even though a character class is
supposed to match just one character of input, perl will match
the whole thing correctly, except when the class is inverted
(C<[^...]>), or the escape is the beginning or final end point of
a range.  For these, what should happen isn't clear at all.  In
these circumstances, Perl discards all but the first character
of the returned sequence, which is not likely what you want.

=item Using just the single character results returned by \p{} in
(?[...]) in regex; marked by S<<-- HERE> in m/%s/

(W regexp) Extended character classes currently cannot handle operands
that evaluate to more than one character.  These are removed from the
results of the expansion of the C<\p{}>.

This situation can happen, for example, in

 (?[ \p{name=/KATAKANA/} ])

"KATAKANA LETTER AINU P" is a legal Unicode name (technically a "named
sequence"), but it is actually two characters.  The above expression
with match only the Unicode names containing KATAKANA that represent
single characters.

=item Using /u for '%s' instead of /%s in regex; marked by S<<-- HERE> in m/%s/

(W regexp) You used a Unicode boundary (C<\b{...}> or C<\B{...}>) in a
portion of a regular expression where the character set modifiers C</a>
or C</aa> are in effect.  These two modifiers indicate an ASCII
interpretation, and this doesn't make sense for a Unicode definition.
The generated regular expression will compile so that the boundary uses
all of Unicode.  No other portion of the regular expression is affected.

=item Using !~ with %s doesn't make sense

(F) Using the C<!~> operator with C<s///r>, C<tr///r> or C<y///r> is
currently reserved for future use, as the exact behavior has not
been decided.  (Simply returning the boolean opposite of the
modified string is usually not particularly useful.)

=item UTF-16 surrogate U+%X

(S surrogate) You had a UTF-16 surrogate in a context where they are
not considered acceptable.  These code points, between U+D800 and
U+DFFF (inclusive), are used by Unicode only for UTF-16.  However, Perl
internally allows all unsigned integer code points (up to the size limit
available on your platform), including surrogates.  But these can cause
problems when being input or output, which is likely where this message
came from.  If you really really know what you are doing you can turn
off this warning by C<no warnings 'surrogate';>.

=item Value of %s can be "0"; test with defined()

(W misc) In a conditional expression, you used <HANDLE>, <*> (glob),
C<each()>, or C<readdir()> as a boolean value.  Each of these constructs
can return a value of "0"; that would make the conditional expression
false, which is probably not what you intended.  When using these
constructs in conditional expressions, test their values with the
C<defined> operator.

=item Value of CLI symbol "%s" too long

(W misc) A warning peculiar to VMS.  Perl tried to read the value of an
%ENV element from a CLI symbol table, and found a resultant string
longer than 1024 characters.  The return value has been truncated to
1024 characters.

=item Variable "%s" is not available

(W closure) During compilation, an inner named subroutine or eval is
attempting to capture an outer lexical that is not currently available.
This can happen for one of two reasons.  First, the outer lexical may be
declared in an outer anonymous subroutine that has not yet been created.
(Remember that named subs are created at compile time, while anonymous
subs are created at run-time.)  For example,

    sub { my $a; sub f { $a } }

At the time that f is created, it can't capture the current value of $a,
since the anonymous subroutine hasn't been created yet.  Conversely,
the following won't give a warning since the anonymous subroutine has by
now been created and is live:

    sub { my $a; eval 'sub f { $a }' }->();

The second situation is caused by an eval accessing a variable that has
gone out of scope, for example,

    sub f {
	my $a;
	sub { eval '$a' }
    }
    f()->();

Here, when the '$a' in the eval is being compiled, f() is not currently
being executed, so its $a is not available for capture.

=item Variable "%s" is not imported%s

(S misc) With "use strict" in effect, you referred to a global variable
that you apparently thought was imported from another module, because
something else of the same name (usually a subroutine) is exported by
that module.  It usually means you put the wrong funny character on the
front of your variable. It is also possible you used an "our" variable
whose scope has ended.

=item Variable length lookbehind not implemented in regex m/%s/

(F) B<This message no longer should be raised as of Perl 5.30.>  It is
retained in this document as a convenience for people using an earlier
Perl version.

In Perl 5.30 and earlier, lookbehind is allowed
only for subexpressions whose length is fixed and
known at compile time.  For positive lookbehind, you can use the C<\K>
regex construct as a way to get the equivalent functionality.  See
L<(?<=pattern) and \K in perlre|perlre/\K>.

Starting in Perl 5.18, there are non-obvious Unicode rules under C</i>
that can match variably, but which you might not think could.  For
example, the substring C<"ss"> can match the single character LATIN
SMALL LETTER SHARP S.  Here's a complete list of the current ones
affecting ASCII characters:

   ASCII
  sequence      Matches single letter under /i
    FF          U+FB00 LATIN SMALL LIGATURE FF
    FFI         U+FB03 LATIN SMALL LIGATURE FFI
    FFL         U+FB04 LATIN SMALL LIGATURE FFL
    FI          U+FB01 LATIN SMALL LIGATURE FI
    FL          U+FB02 LATIN SMALL LIGATURE FL
    SS          U+00DF LATIN SMALL LETTER SHARP S
                U+1E9E LATIN CAPITAL LETTER SHARP S
    ST          U+FB06 LATIN SMALL LIGATURE ST
                U+FB05 LATIN SMALL LIGATURE LONG S T

This list is subject to change, but is quite unlikely to.
Each ASCII sequence can be any combination of upper- and lowercase.

You can avoid this by using a bracketed character class in the
lookbehind assertion, like

 (?<![sS]t)
 (?<![fF]f[iI])

This fools Perl into not matching the ligatures.

Another option for Perls starting with 5.16, if you only care about
ASCII matches, is to add the C</aa> modifier to the regex.  This will
exclude all these non-obvious matches, thus getting rid of this message.
You can also say

 use if $] ge 5.016, re => '/aa';

to apply C</aa> to all regular expressions compiled within its scope.
See L<re>.

=item Variable length positive lookbehind with capturing is experimental in regex m/%s/

(W) Variable length positive lookbehind with capturing is not well defined. This
warning alerts you to the fact that you are using a construct which may
change in a future version of perl. See the
L<< documentation of Positive Lookbehind in perlre|perlre/"C<(?<=I<pattern>)>" >>
for details. You may silence this warning with the following:

    no warnings 'experimental::vlb';

=item Variable length negative lookbehind with capturing is experimental in regex m/%s/

(W) Variable length negative lookbehind with capturing is not well defined. This
warning alerts you to the fact that you are using a construct which may
change in a future version of perl. See the
L<< documentation of Negative Lookbehind in perlre|perlre/"C<(?<!I<pattern>)>" >>
for details. You may silence this warning with the following:

    no warnings 'experimental::vlb';

=item "%s" variable %s masks earlier declaration in same %s

(W shadow) A "my", "our" or "state" variable has been redeclared in the
current scope or statement, effectively eliminating all access to the
previous instance.  This is almost always a typographical error.  Note
that the earlier variable will still exist until the end of the scope
or until all closure references to it are destroyed.

=item Variable syntax

(A) You've accidentally run your script through B<csh> instead
of Perl.  Check the #! line, or manually feed your script into
Perl yourself.

=item Variable "%s" will not stay shared

(W closure) An inner (nested) I<named> subroutine is referencing a
lexical variable defined in an outer named subroutine.

When the inner subroutine is called, it will see the value of
the outer subroutine's variable as it was before and during the *first*
call to the outer subroutine; in this case, after the first call to the
outer subroutine is complete, the inner and outer subroutines will no
longer share a common value for the variable.  In other words, the
variable will no longer be shared.

This problem can usually be solved by making the inner subroutine
anonymous, using the C<sub {}> syntax.  When inner anonymous subs that
reference variables in outer subroutines are created, they
are automatically rebound to the current values of such variables.

=item vector argument not supported with alpha versions

(S printf) The %vd (s)printf format does not support version objects
with alpha parts.

=item Verb pattern '%s' has a mandatory argument in regex; marked by
S<<-- HERE> in m/%s/ 

(F) You used a verb pattern that requires an argument.  Supply an
argument or check that you are using the right verb.

=item Verb pattern '%s' may not have an argument in regex; marked by
S<<-- HERE> in m/%s/ 

(F) You used a verb pattern that is not allowed an argument.  Remove the 
argument or check that you are using the right verb.

=item Version control conflict marker

(F) The parser found a line starting with C<E<lt><<<<<<>,
C<E<gt>E<gt>E<gt>E<gt>E<gt>E<gt>E<gt>>, or C<=======>.  These may be left by a
version control system to mark conflicts after a failed merge operation.

=item Version number must be a constant number

(P) The attempt to translate a C<use Module n.n LIST> statement into
its equivalent C<BEGIN> block found an internal inconsistency with
the version number.

=item Version string '%s' contains invalid data; ignoring: '%s'

(W misc) The version string contains invalid characters at the end, which
are being ignored.

=item Warning: something's wrong

(W) You passed warn() an empty string (the equivalent of C<warn "">) or
you called it with no args and C<$@> was empty.

=item Warning: unable to close filehandle %s properly

(S) The implicit close() done by an open() got an error indication on
the close().  This usually indicates your file system ran out of disk
space.

=item Warning: unable to close filehandle properly: %s

=item Warning: unable to close filehandle %s properly: %s

(S io) There were errors during the implicit close() done on a filehandle
when its reference count reached zero while it was still open, e.g.:

    {
        open my $fh, '>', $file  or die "open: '$file': $!\n";
        print $fh $data or die "print: $!";
    } # implicit close here

Because various errors may only be detected by close() (e.g. buffering could
allow the C<print> in this example to return true even when the disk is full),
it is dangerous to ignore its result.  So when it happens implicitly, perl
will signal errors by warning.

B<Prior to version 5.22.0, perl ignored such errors>, so the common idiom shown
above was liable to cause B<silent data loss>.

=item Warning: Use of "%s" without parentheses is ambiguous

(S ambiguous) You wrote a unary operator followed by something that
looks like a binary operator that could also have been interpreted as a
term or unary operator.  For instance, if you know that the rand
function has a default argument of 1.0, and you write

    rand + 5;

you may THINK you wrote the same thing as

    rand() + 5;

but in actual fact, you got

    rand(+5);

So put in parentheses to say what you really mean.

=item when is deprecated

(D deprecated::smartmatch) C<when> depends on smartmatch, which is
deprecated.  Additionally, it has several special cases that may
not be immediately obvious, and it will be removed in Perl 5.42.
See the explanation
under L<perlsyn/Experimental Details on given and when>.

=item Wide character in %s

(S utf8) Perl met a wide character (ordinal >255) when it wasn't
expecting one.  This warning is by default on for I/O (like print).

If this warning does come from I/O, the easiest
way to quiet it is simply to add the C<:utf8> layer, I<e.g.>,
S<C<binmode STDOUT, ':utf8'>>.  Another way to turn off the warning is
to add S<C<no warnings 'utf8';>> but that is often closer to
cheating.  In general, you are supposed to explicitly mark the
filehandle with an encoding, see L<open> and L<perlfunc/binmode>.

If the warning comes from other than I/O, this diagnostic probably
indicates that incorrect results are being obtained.  You should examine
your code to determine how a wide character is getting to an operation
that doesn't handle them.

=item Wide character (U+%X) in %s

(W locale) While in a single-byte locale (I<i.e.>, a non-UTF-8
one), a multi-byte character was encountered.   Perl considers this
character to be the specified Unicode code point.  Combining non-UTF-8
locales and Unicode is dangerous.  Almost certainly some characters
will have two different representations.  For example, in the ISO 8859-7
(Greek) locale, the code point 0xC3 represents a Capital Gamma.  But so
also does 0x393.  This will make string comparisons unreliable.

You likely need to figure out how this multi-byte character got mixed up
with your single-byte locale (or perhaps you thought you had a UTF-8
locale, but Perl disagrees).

=item Within []-length '%c' not allowed

(F) The count in the (un)pack template may be replaced by C<[TEMPLATE]>
only if C<TEMPLATE> always matches the same amount of packed bytes that
can be determined from the template alone.  This is not possible if
it contains any of the codes @, /, U, u, w or a *-length.  Redesign
the template.

=item While trying to resolve method call %s->%s() can not locate package "%s" yet it is mentioned in @%s::ISA (perhaps you forgot to load "%s"?)

(W syntax) It is possible that the C<@ISA> contains a misspelled or never loaded
package name, which can result in perl choosing an unexpected parent
class's method to resolve the method call. If this is deliberate you
can do something like

  @Missing::Package::ISA = ();

to silence the warnings, otherwise you should correct the package name, or
ensure that the package is loaded prior to the method call.

=item %s() with negative argument

(S misc) Certain operations make no sense with negative arguments.
Warning is given and the operation is not done.

=item write() on closed filehandle %s

(W closed) The filehandle you're writing to got itself closed sometime
before now.  Check your control flow.

=item %s "\x%X" does not map to Unicode

(S utf8) When reading in different encodings, Perl tries to
map everything into Unicode characters.  The bytes you read
in are not legal in this encoding.  For example

    utf8 "\xE4" does not map to Unicode

if you try to read in the a-diaereses Latin-1 as UTF-8.

=item 'X' outside of string

(F) You had a (un)pack template that specified a relative position before
the beginning of the string being (un)packed.  See L<perlfunc/pack>.

=item 'x' outside of string in unpack

(F) You had an unpack template that specified a relative position after
the end of the string being unpacked.  See L<perlfunc/pack>.

=item YOU HAVEN'T DISABLED SET-ID SCRIPTS IN THE KERNEL YET!

(F) And you probably never will, because you probably don't have the
sources to your kernel, and your vendor probably doesn't give a rip
about what you want.  There is a vulnerability anywhere that you have a
set-id script, and to close it you need to remove the set-id bit from
the script that you're attempting to run.  To actually run the script
set-id, your best bet is to put a set-id C wrapper around your script.

=item You need to quote "%s"

(W syntax) You assigned a bareword as a signal handler name.
Unfortunately, you already have a subroutine of that name declared,
which means that Perl 5 will try to call the subroutine when the
assignment is executed, which is probably not what you want.  (If it IS
what you want, put an & in front.)

=item Your random numbers are not that random

(F) When trying to initialize the random seed for hashes, Perl could
not get any randomness out of your system.  This usually indicates
Something Very Wrong.

=item Zero length \N{} in regex; marked by S<<-- HERE> in m/%s/

(F) Named Unicode character escapes (C<\N{...}>) may return a zero-length
sequence.  Such an escape was used in an extended character class, i.e.
C<(?[...])>, or under C<use re 'strict'>, which is not permitted.  Check
that the correct escape has been used, and the correct charnames handler
is in scope.  The S<<-- HERE> shows whereabouts in the regular
expression the problem was discovered.

=back

=head1 SEE ALSO

L<warnings>, L<diagnostics>.

=cut
