Increase $Net::Ping::VERSION to 2.42

[perl5.git] / pod / perldiag.pod

=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 Allocation too large: %x

(X) You can't allocate more than 64K on an MS-DOS machine.

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

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

=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 %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 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 '|' 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 %s argument is not a HASH or ARRAY element or a subroutine

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

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

=item %s argument is not a HASH or ARRAY element or slice

(F) The argument to 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"}

=item %s argument is not a subroutine name

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

=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.

=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 Array @%s missing the @ in argument %d of %s()

(D deprecated) Really old Perl let you omit the @ on array names in some
spots.  This is now heavily deprecated.

=item A sequence of multiple spaces in a charnames alias definition is deprecated

(D) 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 assertion botched: %s

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

=item Assertion failed: file "%s"

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

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

(F) When the "array_base" feature is disabled (e.g., under C<use v5.16;>)
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 A thread exited while %d threads were running

(W threads)(S) When using threaded Perl, a thread (not necessarily
the main thread) exited while there were still other threads running.
Usually it's a good idea first to collect the return values of the
created threads by joining them, and only then to exit from the main
thread.  See L<threads>.

=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 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 join self

(F) You tried to join a thread from within itself, which is an
impossible task.  You may be joining the wrong thread, or you may need
to move the join() to some other thread.

=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 "locked" is deprecated

(D deprecated) You have used the attributes pragma to modify the
"locked" attribute on a code reference.  The :locked attribute is
obsolete, has had no effect since 5005 threads were removed, and
will be removed in a future release of Perl 5.

=item Attribute "unique" is deprecated

(D deprecated) You have used the attributes pragma to modify
the "unique" attribute on an array, hash or scalar reference.
The :unique attribute has had no effect since Perl 5.8.8, and
will be removed in a future release of Perl 5.

=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 hash

(P) One of the internal hash routines was passed a null HV pointer.

=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 array

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

=item Bad symbol for dirhandle

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

=item Bad symbol for filehandle

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

=item Bad symbol for hash

(P) An internal request asked to add a hash 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 "%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 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 \1 better written as $1

(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 "\b{" is deprecated; use "\b\{" or "\b[{]" instead in regex; marked by <-- HERE in m/%s/

=item "\B{" is deprecated; use "\B\{" or "\B[{]" instead in regex; marked by <-- HERE in m/%s/

(W deprecated) Use of an unescaped "{" immediately following a
C<\b> or C<\B> is now deprecated so as to reserve its use for Perl
itself in a future release.  You can either precede the brace with a
backslash, or enclose it in square brackets; the latter is the way to go
if the pattern delimiters are C<{}>.

=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 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 Bizarre SvTYPE [%d]

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

=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 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 Infinity or 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 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 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 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 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 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 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 check filesystem of script "%s" for nosuid

(P) For some reason you can't check the filesystem of the script for
nosuid.

=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 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 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 without backup

(F) You're on a system such as MS-DOS that gets confused if you try
reading from a deleted (but still opened) file.  You have to say
C<-i.bak>, or some such.

=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 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 %s character property "%s"

(F) You used C<\p{}> or C<\P{}> but the character property by that name
could not be found.  Maybe you misspelled the name of the property?
See L<perluniprops/Properties accessible through \p{} and \P{}>
for a complete list of available official properties.

=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"

(F) You may have tried to use C<\p> which means a Unicode
property (for example C<\p{Lu}> matches all uppercase
letters).  If you did mean to use a Unicode property, see
L<perluniprops/Properties accessible through \p{} and \P{}>
for a complete list of available properties.  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 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 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 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 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 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

(W) 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 nonexistent substring

(P) The internal routine that does assignment to a substr() was handed
a NULL.

=item Can't modify non-lvalue subroutine call

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

=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

(F) You tried to run a perl built with MAD support with
the PERL_XMLDUMP environment variable set, but the file
named by that variable could not be opened.

=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 "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 %s to %s: %s, skipping file

(S inplace) 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 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 stat script "%s"

(P) For some reason you can't fstat() the script even though you have it
open already.  Bizarre.

=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 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 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 %s for loop variable

(F) Only a simple scalar variable may be used as a loop variable on a
foreach.

=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

(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 Character following "\c" must be ASCII

(F)(W deprecated, syntax) In C<\cI<X>>, I<X> must be an ASCII character.
It is planned to make this fatal in all instances in Perl v5.20.  In the
cases where it isn't fatal, the character this evaluates to is
derived by exclusive or'ing the code point of this character with 0x40.

Note that non-alphabetic ASCII characters are discouraged here as well,
and using non-printable ones will be deprecated starting in v5.18.

=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 '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 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(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 "\c{" is deprecated and is more clearly written as ";"

(D deprecated, syntax) The C<\cI<X>> construct is intended to be a way
to specify non-printable characters.  You used it with a "{" which
evaluates to ";", which is printable.  It is planned to remove the
ability to specify a semi-colon this way in Perl 5.20.  Just use a
semi-colon or a backslash-semi-colon without the "\c".

=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.

=item Cloning substitution context is unimplemented

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

=item close() on unopened filehandle %s

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

=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 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 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, may not be portable

=item Code point 0x%X is not Unicode, all \p{} matches fail; all \P{} matches 
succeed

(S utf8, 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, up
to the limit of what is storable in an unsigned integer on your system,
but these may not be accepted by other languages/systems.  At one time,
it was legal in some standards to have code points up to 0x7FFF_FFFF,
but not higher.  Code points above 0xFFFF_FFFF require larger than a
32 bit word.

None of the Unicode or Perl-defined properties will match a non-Unicode
code point.  For example,

    chr(0x7FF_FFFF) =~ /\p{Any}/

will not match, because the code point is not in Unicode.  But

    chr(0x7FF_FFFF) =~ /\P{Any}/

will match.

This may be counterintuitive at times, as both these fail:

 chr(0x110000) =~ /\p{ASCII_Hex_Digit=True}/      # Fails.
 chr(0x110000) =~ /\p{ASCII_Hex_Digit=False}/     # Also fails!

and both these succeed:

 chr(0x110000) =~ /\P{ASCII_Hex_Digit=True}/      # Succeeds.
 chr(0x110000) =~ /\P{ASCII_Hex_Digit=False}/     # Also succeeds!

=item %s: Command not found

(A) You've accidentally run your script through B<csh> or another shell
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 -w

=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 Complex regular subexpression recursion limit (%d) exceeded

(W regexp) The regular expression engine uses recursion in complex
situations where back-tracking is required.  Recursion depth is limited
to 32766, or perhaps less in architectures where the stack cannot grow
arbitrarily.  ("Simple" and "medium" situations are handled without
recursion and are not subject to a limit.)  Try shortening the string
under examination; looping in Perl code (e.g. with C<while>) rather than
in the regular expression engine; or rewriting the regular expression so
that it is simpler or backtracks less.  (See L<perlfaq2> for information
on I<Mastering Regular Expressions>.)

=item cond_broadcast() called on unlocked variable

(W threads) Within a thread-enabled program, you tried to
call cond_broadcast() on a variable which wasn't locked.
The cond_broadcast() function is used to wake up another thread
that is waiting in a cond_wait().  To ensure that the signal isn't
sent before the other thread has a chance to enter the wait, it
is usual for the signaling thread first to wait for a lock on
variable.  This lock attempt will only succeed after the other
thread has entered cond_wait() and thus relinquished the lock.

=item cond_signal() called on unlocked variable

(W threads) Within a thread-enabled program, you tried to
call cond_signal() on a variable which wasn't locked.  The
cond_signal() function is used to wake up another thread that
is waiting in a cond_wait().  To ensure that the signal isn't
sent before the other thread has a chance to enter the wait, it
is usual for the signaling thread first to wait for a lock on
variable.  This lock attempt will only succeed after the other
thread has entered cond_wait() and thus relinquished the lock.

=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(%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 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 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 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 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 Corrupted regexp opcode %d > %d

(F)
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 through the L<perlbug> utility.

=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 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 defined(@array) is deprecated

(D deprecated) defined() is not usually 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 defined(%hash) is deprecated

(D deprecated) C<defined()> is not usually right on hashes and has been
discouraged since 5.004.

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.

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 (?(DEFINE)....) does not allow branches in regex; marked by <-- 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 <-- 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 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 Deprecated use of my() in false conditional

(D deprecated) 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 5.9.4, you can also use C<state> variables to have
lexicals that are initialized only once (see L<feature>):

    sub f { state $x; return $x++ }

=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 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 misc) 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 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

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

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

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

=item do_study: out of memory

(P) This should have been caught by safemalloc() instead.

=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() better written as CORE::dump()

(W misc) You used the obsolescent C<dump()> built-in function, without fully
qualifying it as C<CORE::dump()>.  Maybe it's a typo.  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 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 <-- HERE in m/%s/

(F) C<\p> and C<\P> are used to introduce a named Unicode property, as
described in L<perlunicode> and L<perlre>.  You used C<\p> or C<\P> in
a regular expression without specifying the property name.

=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 Escape literal pattern white space under /x

(D deprecated) You compiled a regular expression pattern with C</x> to
ignore white space, and you used, as a literal, one of the characters
that Perl plans to eventually treat as white space.  The character must
be escaped somehow, or it will work differently on a future Perl that
does treat it as white space.  The easiest way is to insert a backslash
immediately before it, or to enclose it with square brackets.  This
change is to bring Perl into conformance with Unicode recommendations.
Here are the five characters that generate this warning:
U+0085 NEXT LINE,
U+200E LEFT-TO-RIGHT MARK,
U+200F RIGHT-TO-LEFT MARK,
U+2028 LINE SEPARATOR,
and
U+2029 PARAGRAPH SEPARATOR.

=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 <-- 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 <-- 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., Symbian
OS.  See L<perlport>.

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

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

=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 Expecting close bracket in regex; marked by <-- 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 Experimental "%s" subs not enabled

(F) To use lexical subs, you must first enable them:

    no warnings 'experimental::lexical_subs';
    use feature 'lexical_subs';
    my sub foo { ... }

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

(W regexp) 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 "-".  Consider quoting the
"-", "\-".  The <-- 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 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 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 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 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 experimental

(S experimental::smartmatch) C<given> depends on both a lexical C<$_> and
smartmatch, both of which are experimental, so its behavior may change or
even be removed in any future release of perl.
See the explanation under L<perlsyn/Experimental Details on given and when>.

=item Global symbol "%s" requires explicit package name

(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) 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 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 Group name must start with a non-digit word character in regex; marked by 
<-- 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 %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 Hash %%s missing the % in argument %d of %s()

(D deprecated) Really old Perl let you omit the % on hash names in some
spots.  This is now heavily deprecated.

=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 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 -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 inplace, but no files were
given.  This is usually a mistake, since editing STDIN inplace 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> for more details.

=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 <-- 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
behaviour is not well defined.  Check that the correct escape has
been used, and the correct charname handler is in scope.

=item Illegal binary digit %s

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

=item Illegal binary digit %s ignored

(W digit) You may have tried to use a digit other than 0 or 1 in a
binary number.  Interpretation of the binary number stopped before the
offending digit.

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

(W illegalproto) An illegal character was found in a prototype declaration.
Legal characters in prototypes are $, @, %, *, ;, [, ], &, \, and +.

=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 in prototype for %s : %s

(W illegalproto) An illegal character was found in a prototype declaration.
Legal characters in prototypes are $, @, %, *, ;, [, ], &, \, and +.

=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 hexadecimal digit %s ignored

(W digit) You may have tried to use a character other than 0 - 9 or
A - F, a - f in a hexadecimal number.  Interpretation of the hexadecimal
number stopped before the illegal character.

=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 %s

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

=item Illegal octal digit %s ignored

(W digit) You may have tried to use an 8 or 9 in an octal number.
Interpretation of the octal number stopped before the 8 or 9.

=item Illegal pattern in regex; marked by <-- 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 switch in PERL5OPT: -%c

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

=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 In '(*VERB...)', splitting the initial '(*' is deprecated in regex; marked by <-- HERE in m/%s/

(D regexp, deprecated)
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.
Due to an accident of implementation, this prohibition was not enforced,
but we do plan to forbid it in a future Perl version.  This message
serves as giving you fair warning of this pending change.

=item In '(?...)', splitting the initial '(?' is deprecated in regex; marked by <-- HERE in m/%s/

(D regexp, deprecated)
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.
Due to an accident of implementation, this prohibition was not enforced,
but we do plan to forbid it in a future Perl version.  This message
serves as giving you fair warning of this pending change.

=item Incomplete expression within '(?[ ])' in regex; marked by <-- 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 In EBCDIC the v-string components cannot exceed 2147483647

(F) An error peculiar to EBCDIC.  Internally, v-strings are stored as
Unicode code points, and encoded in EBCDIC as UTF-EBCDIC.  The UTF-EBCDIC
encoding is limited to code points no larger than 2147483647 (0x7FFFFFFF).

=item Infinite recursion in regex; marked by <-- HERE in m/%s/

(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.

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

=item Initialization of state variables in list context currently forbidden

(F) Currently the implementation of "state" only permits the
initialization of scalar variables in scalar context.  Re-write
C<state ($a) = 42> as C<state $a = 42> to change from list to scalar
context.  Constructions such as C<state (@a) = foo()> will be
supported in a future perl release.

=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 <-- HERE in m/%s/

(P) Something went badly wrong in the regular expression parser.
The <-- 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 urp in regex; marked by <-- HERE in m/%s/

(P) Something went badly awry in the regular expression parser.  The
<-- 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 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 [] range "%*.*s" in regex; marked by <-- HERE in m/%s/

(F)
You wrote something like

 [z-a]

in a regular expression pattern.  Ranges must be specified with the
lowest code point first.  Instead write

 [a-z]

=item Invalid character in \N{...}; marked by <-- 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 character in charnames alias definition; marked by <-- 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 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 <-- HERE in 
m/%s/

(W regexp) 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.
The <-- 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 <-- 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 characters numbers, so it return the Unicode replacement
character (U+FFFD).

=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 [] range "%s" in regex; marked by <-- 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 <-- 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 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 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 $* is no longer supported

(D deprecated, syntax) The special variable C<$*>, deprecated in older
perls, has been removed as of 5.9.0 and is no longer supported.  In
previous versions of perl the use of C<$*> 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>.)

=item $# is no longer supported

(D deprecated, syntax) The special variable C<$#>, deprecated in older
perls, has been removed as of 5.9.3 and is no longer supported.  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 Junk on end of regexp in regex m/%s/

(P) The regular expression parser is confused.

=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

(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 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) The value you attempted to increment or decrement by one
is too large for the underlying floating point representation to store
accurately, hence the target of C<++> or C<--> is unchanged.  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 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.

=item Malformed UTF-8 character (%s)

(S utf8)(F) Perl detected a string that didn't comply with UTF-8
encoding rules, even though it had the UTF8 flag on.

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 Encode::decode_utf8.

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 character immediately after '%s'

(F) You said C<use utf8>, but the program file doesn't comply with UTF-8
encoding rules.  The message prints out the properly encoded characters
just before the first bad one.  If C<utf8> warnings are enabled, a
warning is generated that gives more details about the type of
malformation.

=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 '%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 %s matches null string many times in regex; marked by <-- 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 <-- 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 %s not permitted

See Server error.

=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 Misplaced _ in number

(W syntax) An underscore (underbar) in a numeric constant did not
separate two digits.

=item Missing argument in %s

(W uninitialized) A printf-type format required more arguments than were
supplied.

=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 <-- 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 name in "%s sub"

(F) The reserved 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 right brace on \%c{} in regex; marked by <-- HERE in m/%s/

(F) Missing right brace in C<\x{...}>, C<\p{...}>, C<\P{...}>, or C<\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

=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> 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 msg%s not implemented

(F) You don't have System V message IPC on your system.

=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 '/' 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 "my sub" not yet implemented

(F) Lexically scoped subroutines are not yet implemented.  Don't try
that yet.

=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
provided for this purpose.

NOTE: This warning detects symbols that have been used only once so $c, @c,
%c, *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.

=item \N in a character class must be a named character: \N{...} in regex; 
marked by <-- HERE in m/%s/

(F) The new (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{NAME} must be resolved by the lexer in regex; marked by <-- HERE in m/%s/

(F) When compiling a regex pattern, an unresolved named character or
sequence was encountered.  This can happen in any of several ways that
bypass the lexer, such as using single-quotish context, or an extra
backslash in double-quotish:

    $re = '\N{SPACE}';	# Wrong!
    $re = "\\N{SPACE}";	# Wrong!
    /$re/;

Instead, use double-quotes with a single backslash:

    $re = "\N{SPACE}";	# ok
    /$re/;

The lexer can be bypassed as well by creating the pattern from smaller
components:

    $re = '\N';
    /${re}{SPACE}/;	# Wrong!

It's not a good idea to split a construct in the middle like this, and it
doesn't work here.  Instead use the solution above.

Finally, the message also can happen under the C</x> regex modifier when the
C<\N> is separated by spaces from the C<{>, in which case, remove the spaces.

    /\N {SPACE}/x;	# Wrong!
    /\N{SPACE}/x;	# ok

=item Need exactly 3 octal digits in regex; marked by <-- 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 Nested quantifiers in regex; marked by <-- HERE in m/%s/

(F) You can't quantify a quantifier without intervening parentheses.
So things like ** or +* or ?* are illegal.  The <-- 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 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 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 "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 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 variable %s in "our"

(F) Fully qualified variable names are not allowed in "our"
declarations, because that doesn't make much sense under existing
semantics.  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.