3 perlvar - Perl predefined variables
7 =head2 Predefined Names
9 The following names have special meaning to Perl. Most
10 punctuation names have reasonable mnemonics, or analogs in the
11 shells. Nevertheless, if you wish to use long variable names,
16 at the top of your program. This aliases all the short names to the long
17 names in the current package. Some even have medium names, generally
18 borrowed from B<awk>. In general, it's best to use the
20 use English '-no_match_vars';
22 invocation if you don't need $PREMATCH, $MATCH, or $POSTMATCH, as it avoids
23 a certain performance hit with the use of regular expressions. See
26 Variables that depend on the currently selected filehandle may be set by
27 calling an appropriate object method on the IO::Handle object, although
28 this is less efficient than using the regular built-in variables. (Summary
29 lines below for this contain the word HANDLE.) First you must say
33 after which you may use either
41 Each method returns the old value of the IO::Handle attribute.
42 The methods each take an optional EXPR, which, if supplied, specifies the
43 new value for the IO::Handle attribute in question. If not supplied,
44 most methods do nothing to the current value--except for
45 autoflush(), which will assume a 1 for you, just to be different.
47 Because loading in the IO::Handle class is an expensive operation, you should
48 learn how to use the regular built-in variables.
50 A few of these variables are considered "read-only". This means that if
51 you try to assign to this variable, either directly or indirectly through
52 a reference, you'll raise a run-time exception.
54 You should be very careful when modifying the default values of most
55 special variables described in this document. In most cases you want
56 to localize these variables before changing them, since if you don't,
57 the change may affect other modules which rely on the default values
58 of the special variables that you have changed. This is one of the
59 correct ways to read the whole file at once:
61 open my $fh, "foo" or die $!;
62 local $/; # enable localized slurp mode
66 But the following code is quite bad:
68 open my $fh, "foo" or die $!;
69 undef $/; # enable slurp mode
73 since some other module, may want to read data from some file in the
74 default "line mode", so if the code we have just presented has been
75 executed, the global value of C<$/> is now changed for any other code
76 running inside the same Perl interpreter.
78 Usually when a variable is localized you want to make sure that this
79 change affects the shortest scope possible. So unless you are already
80 inside some short C<{}> block, you should create one yourself. For
84 open my $fh, "foo" or die $!;
91 Here is an example of how your own code can go broken:
99 # do something with $_
102 You probably expect this code to print:
110 Why? Because nasty_break() modifies C<$_> without localizing it
111 first. The fix is to add local():
115 It's easy to notice the problem in such a short example, but in more
116 complicated code you are looking for trouble if you don't localize
117 changes to the special variables.
119 The following list is ordered by scalar variables first, then the
120 arrays, then the hashes.
129 The default input and pattern-searching space. The following pairs are
132 while (<>) {...} # equivalent only in while!
133 while (defined($_ = <>)) {...}
144 Here are the places where Perl will assume $_ even if you
151 Various unary functions, including functions like ord() and int(), as well
152 as the all file tests (C<-f>, C<-d>) except for C<-t>, which defaults to
157 Various list functions like print() and unlink().
161 The pattern matching operations C<m//>, C<s///>, and C<tr///> when used
162 without an C<=~> operator.
166 The default iterator variable in a C<foreach> loop if no other
167 variable is supplied.
171 The implicit iterator variable in the grep() and map() functions.
175 The default place to put an input record when a C<< <FH> >>
176 operation's result is tested by itself as the sole criterion of a C<while>
177 test. Outside a C<while> test, this will not happen.
181 As C<$_> is a global variable, this may lead in some cases to unwanted
182 side-effects. As of perl 5.9.1, you can now use a lexical version of
183 C<$_> by declaring it in a file or in a block with C<my>. Moreover,
184 declaring C<our $_> restores the global C<$_> in the current scope.
186 (Mnemonic: underline is understood in certain operations.)
197 Special package variables when using sort(), see L<perlfunc/sort>.
198 Because of this specialness $a and $b don't need to be declared
199 (using use vars, or our()) even when using the C<strict 'vars'> pragma.
200 Don't lexicalize them with C<my $a> or C<my $b> if you want to be
201 able to use them in the sort() comparison block or function.
210 Contains the subpattern from the corresponding set of capturing
211 parentheses from the last pattern match, not counting patterns
212 matched in nested blocks that have been exited already. (Mnemonic:
213 like \digits.) These variables are all read-only and dynamically
214 scoped to the current BLOCK.
221 The string matched by the last successful pattern match (not counting
222 any matches hidden within a BLOCK or eval() enclosed by the current
223 BLOCK). (Mnemonic: like & in some editors.) This variable is read-only
224 and dynamically scoped to the current BLOCK.
226 The use of this variable anywhere in a program imposes a considerable
227 performance penalty on all regular expression matches. See L</BUGS>.
229 See L</@-> for a replacement.
234 This is similar to C<$&> (C<$POSTMATCH>) except that it does not incur the
235 performance penalty associated with that variable, and is only guaranteed
236 to return a defined value when the pattern was compiled or executed with
244 The string preceding whatever was matched by the last successful
245 pattern match (not counting any matches hidden within a BLOCK or eval
246 enclosed by the current BLOCK). (Mnemonic: C<`> often precedes a quoted
247 string.) This variable is read-only.
249 The use of this variable anywhere in a program imposes a considerable
250 performance penalty on all regular expression matches. See L</BUGS>.
252 See L</@-> for a replacement.
257 This is similar to C<$`> ($PREMATCH) except that it does not incur the
258 performance penalty associated with that variable, and is only guaranteed
259 to return a defined value when the pattern was compiled or executed with
267 The string following whatever was matched by the last successful
268 pattern match (not counting any matches hidden within a BLOCK or eval()
269 enclosed by the current BLOCK). (Mnemonic: C<'> often follows a quoted
272 local $_ = 'abcdefghi';
274 print "$`:$&:$'\n"; # prints abc:def:ghi
276 This variable is read-only and dynamically scoped to the current BLOCK.
278 The use of this variable anywhere in a program imposes a considerable
279 performance penalty on all regular expression matches. See L</BUGS>.
281 See L</@-> for a replacement.
286 This is similar to C<$'> (C<$POSTMATCH>) except that it does not incur the
287 performance penalty associated with that variable, and is only guaranteed
288 to return a defined value when the pattern was compiled or executed with
291 =item $LAST_PAREN_MATCH
294 X<$+> X<$LAST_PAREN_MATCH>
296 The text matched by the last bracket of the last successful search pattern.
297 This is useful if you don't know which one of a set of alternative patterns
298 matched. For example:
300 /Version: (.*)|Revision: (.*)/ && ($rev = $+);
302 (Mnemonic: be positive and forward looking.)
303 This variable is read-only and dynamically scoped to the current BLOCK.
305 =item $LAST_SUBMATCH_RESULT
310 The text matched by the used group most-recently closed (i.e. the group
311 with the rightmost closing parenthesis) of the last successful search
312 pattern. (Mnemonic: the (possibly) Nested parenthesis that most
315 This is primarily used inside C<(?{...})> blocks for examining text
316 recently matched. For example, to effectively capture text to a variable
317 (in addition to C<$1>, C<$2>, etc.), replace C<(...)> with
319 (?:(...)(?{ $var = $^N }))
321 By setting and then using C<$var> in this way relieves you from having to
322 worry about exactly which numbered set of parentheses they are.
324 This variable is dynamically scoped to the current BLOCK.
326 =item @LAST_MATCH_END
329 X<@+> X<@LAST_MATCH_END>
331 This array holds the offsets of the ends of the last successful
332 submatches in the currently active dynamic scope. C<$+[0]> is
333 the offset into the string of the end of the entire match. This
334 is the same value as what the C<pos> function returns when called
335 on the variable that was matched against. The I<n>th element
336 of this array holds the offset of the I<n>th submatch, so
337 C<$+[1]> is the offset past where $1 ends, C<$+[2]> the offset
338 past where $2 ends, and so on. You can use C<$#+> to determine
339 how many subgroups were in the last successful match. See the
340 examples given for the C<@-> variable.
345 Similar to C<@+>, the C<%+> hash allows access to the named capture
346 buffers, should they exist, in the last successful match in the
347 currently active dynamic scope.
349 For example, C<$+{foo}> is equivalent to C<$1> after the following match:
351 'foo' =~ /(?<foo>foo)/;
353 The keys of the C<%+> hash list only the names of buffers that have
354 captured (and that are thus associated to defined values).
356 The underlying behaviour of C<%+> is provided by the
357 L<Tie::Hash::NamedCapture> module.
359 B<Note:> C<%-> and C<%+> are tied views into a common internal hash
360 associated with the last successful regular expression. Therefore mixing
361 iterative access to them via C<each> may have unpredictable results.
362 Likewise, if the last successful match changes, then the results may be
365 =item HANDLE->input_line_number(EXPR)
367 =item $INPUT_LINE_NUMBER
372 X<$.> X<$NR> X<$INPUT_LINE_NUMBER> X<line number>
374 Current line number for the last filehandle accessed.
376 Each filehandle in Perl counts the number of lines that have been read
377 from it. (Depending on the value of C<$/>, Perl's idea of what
378 constitutes a line may not match yours.) When a line is read from a
379 filehandle (via readline() or C<< <> >>), or when tell() or seek() is
380 called on it, C<$.> becomes an alias to the line counter for that
383 You can adjust the counter by assigning to C<$.>, but this will not
384 actually move the seek pointer. I<Localizing C<$.> will not localize
385 the filehandle's line count>. Instead, it will localize perl's notion
386 of which filehandle C<$.> is currently aliased to.
388 C<$.> is reset when the filehandle is closed, but B<not> when an open
389 filehandle is reopened without an intervening close(). For more
390 details, see L<perlop/"IE<sol>O Operators">. Because C<< <> >> never does
391 an explicit close, line numbers increase across ARGV files (but see
392 examples in L<perlfunc/eof>).
394 You can also use C<< HANDLE->input_line_number(EXPR) >> to access the
395 line counter for a given filehandle without having to worry about
396 which handle you last accessed.
398 (Mnemonic: many programs use "." to mean the current line number.)
400 =item IO::Handle->input_record_separator(EXPR)
402 =item $INPUT_RECORD_SEPARATOR
407 X<$/> X<$RS> X<$INPUT_RECORD_SEPARATOR>
409 The input record separator, newline by default. This
410 influences Perl's idea of what a "line" is. Works like B<awk>'s RS
411 variable, including treating empty lines as a terminator if set to
412 the null string. (An empty line cannot contain any spaces
413 or tabs.) You may set it to a multi-character string to match a
414 multi-character terminator, or to C<undef> to read through the end
415 of file. Setting it to C<"\n\n"> means something slightly
416 different than setting to C<"">, if the file contains consecutive
417 empty lines. Setting to C<""> will treat two or more consecutive
418 empty lines as a single empty line. Setting to C<"\n\n"> will
419 blindly assume that the next input character belongs to the next
420 paragraph, even if it's a newline. (Mnemonic: / delimits
421 line boundaries when quoting poetry.)
423 local $/; # enable "slurp" mode
424 local $_ = <FH>; # whole file now here
427 Remember: the value of C<$/> is a string, not a regex. B<awk> has to be
428 better for something. :-)
430 Setting C<$/> to a reference to an integer, scalar containing an integer, or
431 scalar that's convertible to an integer will attempt to read records
432 instead of lines, with the maximum record size being the referenced
435 local $/ = \32768; # or \"32768", or \$var_containing_32768
436 open my $fh, $myfile or die $!;
439 will read a record of no more than 32768 bytes from FILE. If you're
440 not reading from a record-oriented file (or your OS doesn't have
441 record-oriented files), then you'll likely get a full chunk of data
442 with every read. If a record is larger than the record size you've
443 set, you'll get the record back in pieces. Trying to set the record
444 size to zero or less will cause reading in the (rest of the) whole file.
446 On VMS, record reads are done with the equivalent of C<sysread>,
447 so it's best not to mix record and non-record reads on the same
448 file. (This is unlikely to be a problem, because any file you'd
449 want to read in record mode is probably unusable in line mode.)
450 Non-VMS systems do normal I/O, so it's safe to mix record and
451 non-record reads of a file.
453 See also L<perlport/"Newlines">. Also see C<$.>.
455 =item HANDLE->autoflush(EXPR)
457 =item $OUTPUT_AUTOFLUSH
460 X<$|> X<autoflush> X<flush> X<$OUTPUT_AUTOFLUSH>
462 If set to nonzero, forces a flush right away and after every write
463 or print on the currently selected output channel. Default is 0
464 (regardless of whether the channel is really buffered by the
465 system or not; C<$|> tells you only whether you've asked Perl
466 explicitly to flush after each write). STDOUT will
467 typically be line buffered if output is to the terminal and block
468 buffered otherwise. Setting this variable is useful primarily when
469 you are outputting to a pipe or socket, such as when you are running
470 a Perl program under B<rsh> and want to see the output as it's
471 happening. This has no effect on input buffering. See L<perlfunc/getc>
472 for that. (Mnemonic: when you want your pipes to be piping hot.)
474 =item IO::Handle->output_field_separator EXPR
476 =item $OUTPUT_FIELD_SEPARATOR
481 X<$,> X<$OFS> X<$OUTPUT_FIELD_SEPARATOR>
483 The output field separator for the print operator. If defined, this
484 value is printed between each of print's arguments. Default is C<undef>.
485 (Mnemonic: what is printed when there is a "," in your print statement.)
487 =item IO::Handle->output_record_separator EXPR
489 =item $OUTPUT_RECORD_SEPARATOR
494 X<$\> X<$ORS> X<$OUTPUT_RECORD_SEPARATOR>
496 The output record separator for the print operator. If defined, this
497 value is printed after the last of print's arguments. Default is C<undef>.
498 (Mnemonic: you set C<$\> instead of adding "\n" at the end of the print.
499 Also, it's just like C<$/>, but it's what you get "back" from Perl.)
501 =item $LIST_SEPARATOR
504 X<$"> X<$LIST_SEPARATOR>
506 This is like C<$,> except that it applies to array and slice values
507 interpolated into a double-quoted string (or similar interpreted
508 string). Default is a space. (Mnemonic: obvious, I think.)
510 =item $SUBSCRIPT_SEPARATOR
515 X<$;> X<$SUBSEP> X<SUBSCRIPT_SEPARATOR>
517 The subscript separator for multidimensional array emulation. If you
518 refer to a hash element as
524 $foo{join($;, $a, $b, $c)}
528 @foo{$a,$b,$c} # a slice--note the @
532 ($foo{$a},$foo{$b},$foo{$c})
534 Default is "\034", the same as SUBSEP in B<awk>. If your
535 keys contain binary data there might not be any safe value for C<$;>.
536 (Mnemonic: comma (the syntactic subscript separator) is a
537 semi-semicolon. Yeah, I know, it's pretty lame, but C<$,> is already
538 taken for something more important.)
540 Consider using "real" multidimensional arrays as described
543 =item HANDLE->format_page_number(EXPR)
545 =item $FORMAT_PAGE_NUMBER
548 X<$%> X<$FORMAT_PAGE_NUMBER>
550 The current page number of the currently selected output channel.
552 (Mnemonic: % is page number in B<nroff>.)
554 =item HANDLE->format_lines_per_page(EXPR)
556 =item $FORMAT_LINES_PER_PAGE
559 X<$=> X<$FORMAT_LINES_PER_PAGE>
561 The current page length (printable lines) of the currently selected
562 output channel. Default is 60.
564 (Mnemonic: = has horizontal lines.)
566 =item HANDLE->format_lines_left(EXPR)
568 =item $FORMAT_LINES_LEFT
571 X<$-> X<$FORMAT_LINES_LEFT>
573 The number of lines left on the page of the currently selected output
576 (Mnemonic: lines_on_page - lines_printed.)
578 =item @LAST_MATCH_START
581 X<@-> X<@LAST_MATCH_START>
583 $-[0] is the offset of the start of the last successful match.
584 C<$-[>I<n>C<]> is the offset of the start of the substring matched by
585 I<n>-th subpattern, or undef if the subpattern did not match.
587 Thus after a match against $_, $& coincides with C<substr $_, $-[0],
588 $+[0] - $-[0]>. Similarly, $I<n> coincides with C<substr $_, $-[n],
589 $+[n] - $-[n]> if C<$-[n]> is defined, and $+ coincides with
590 C<substr $_, $-[$#-], $+[$#-] - $-[$#-]>. One can use C<$#-> to find the last
591 matched subgroup in the last successful match. Contrast with
592 C<$#+>, the number of subgroups in the regular expression. Compare
595 This array holds the offsets of the beginnings of the last
596 successful submatches in the currently active dynamic scope.
597 C<$-[0]> is the offset into the string of the beginning of the
598 entire match. The I<n>th element of this array holds the offset
599 of the I<n>th submatch, so C<$-[1]> is the offset where $1
600 begins, C<$-[2]> the offset where $2 begins, and so on.
602 After a match against some variable $var:
606 =item C<$`> is the same as C<substr($var, 0, $-[0])>
608 =item C<$&> is the same as C<substr($var, $-[0], $+[0] - $-[0])>
610 =item C<$'> is the same as C<substr($var, $+[0])>
612 =item C<$1> is the same as C<substr($var, $-[1], $+[1] - $-[1])>
614 =item C<$2> is the same as C<substr($var, $-[2], $+[2] - $-[2])>
616 =item C<$3> is the same as C<substr($var, $-[3], $+[3] - $-[3])>
623 Similar to C<%+>, this variable allows access to the named capture buffers
624 in the last successful match in the currently active dynamic scope. To
625 each capture buffer name found in the regular expression, it associates a
626 reference to an array containing the list of values captured by all
627 buffers with that name (should there be several of them), in the order
632 if ('1234' =~ /(?<A>1)(?<B>2)(?<A>3)(?<B>4)/) {
633 foreach my $bufname (sort keys %-) {
634 my $ary = $-{$bufname};
635 foreach my $idx (0..$#$ary) {
636 print "\$-{$bufname}[$idx] : ",
637 (defined($ary->[$idx]) ? "'$ary->[$idx]'" : "undef"),
650 The keys of the C<%-> hash correspond to all buffer names found in
651 the regular expression.
653 The behaviour of C<%-> is implemented via the
654 L<Tie::Hash::NamedCapture> module.
656 B<Note:> C<%-> and C<%+> are tied views into a common internal hash
657 associated with the last successful regular expression. Therefore mixing
658 iterative access to them via C<each> may have unpredictable results.
659 Likewise, if the last successful match changes, then the results may be
662 =item HANDLE->format_name(EXPR)
667 X<$~> X<$FORMAT_NAME>
669 The name of the current report format for the currently selected output
670 channel. Default is the name of the filehandle. (Mnemonic: brother to
673 =item HANDLE->format_top_name(EXPR)
675 =item $FORMAT_TOP_NAME
678 X<$^> X<$FORMAT_TOP_NAME>
680 The name of the current top-of-page format for the currently selected
681 output channel. Default is the name of the filehandle with _TOP
682 appended. (Mnemonic: points to top of page.)
684 =item IO::Handle->format_line_break_characters EXPR
686 =item $FORMAT_LINE_BREAK_CHARACTERS
689 X<$:> X<FORMAT_LINE_BREAK_CHARACTERS>
691 The current set of characters after which a string may be broken to
692 fill continuation fields (starting with ^) in a format. Default is
693 S<" \n-">, to break on whitespace or hyphens. (Mnemonic: a "colon" in
694 poetry is a part of a line.)
696 =item IO::Handle->format_formfeed EXPR
698 =item $FORMAT_FORMFEED
701 X<$^L> X<$FORMAT_FORMFEED>
703 What formats output as a form feed. Default is \f.
708 X<$^A> X<$ACCUMULATOR>
710 The current value of the write() accumulator for format() lines. A format
711 contains formline() calls that put their result into C<$^A>. After
712 calling its format, write() prints out the contents of C<$^A> and empties.
713 So you never really see the contents of C<$^A> unless you call
714 formline() yourself and then look at it. See L<perlform> and
715 L<perlfunc/formline()>.
720 X<$?> X<$CHILD_ERROR>
722 The status returned by the last pipe close, backtick (C<``>) command,
723 successful call to wait() or waitpid(), or from the system()
724 operator. This is just the 16-bit status word returned by the
725 traditional Unix wait() system call (or else is made up to look like it). Thus, the
726 exit value of the subprocess is really (C<<< $? >> 8 >>>), and
727 C<$? & 127> gives which signal, if any, the process died from, and
728 C<$? & 128> reports whether there was a core dump. (Mnemonic:
729 similar to B<sh> and B<ksh>.)
731 Additionally, if the C<h_errno> variable is supported in C, its value
732 is returned via $? if any C<gethost*()> function fails.
734 If you have installed a signal handler for C<SIGCHLD>, the
735 value of C<$?> will usually be wrong outside that handler.
737 Inside an C<END> subroutine C<$?> contains the value that is going to be
738 given to C<exit()>. You can modify C<$?> in an C<END> subroutine to
739 change the exit status of your program. For example:
742 $? = 1 if $? == 255; # die would make it 255
745 Under VMS, the pragma C<use vmsish 'status'> makes C<$?> reflect the
746 actual VMS exit status, instead of the default emulation of POSIX
747 status; see L<perlvms/$?> for details.
749 Also see L<Error Indicators>.
751 =item ${^CHILD_ERROR_NATIVE}
752 X<$^CHILD_ERROR_NATIVE>
754 The native status returned by the last pipe close, backtick (C<``>)
755 command, successful call to wait() or waitpid(), or from the system()
756 operator. On POSIX-like systems this value can be decoded with the
757 WIFEXITED, WEXITSTATUS, WIFSIGNALED, WTERMSIG, WIFSTOPPED, WSTOPSIG
758 and WIFCONTINUED functions provided by the L<POSIX> module.
760 Under VMS this reflects the actual VMS exit status; i.e. it is the same
761 as $? when the pragma C<use vmsish 'status'> is in effect.
766 The I<object reference> to the Encode object that is used to convert
767 the source code to Unicode. Thanks to this variable your perl script
768 does not have to be written in UTF-8. Default is I<undef>. The direct
769 manipulation of this variable is highly discouraged.
776 X<$!> X<$ERRNO> X<$OS_ERROR>
778 If used numerically, yields the current value of the C C<errno>
779 variable, or in other words, if a system or library call fails, it
780 sets this variable. This means that the value of C<$!> is meaningful
781 only I<immediately> after a B<failure>:
783 if (open(FH, $filename)) {
784 # Here $! is meaningless.
787 # ONLY here is $! meaningful.
789 # Already here $! might be meaningless.
791 # Since here we might have either success or failure,
792 # here $! is meaningless.
794 In the above I<meaningless> stands for anything: zero, non-zero,
795 C<undef>. A successful system or library call does B<not> set
796 the variable to zero.
798 If used as a string, yields the corresponding system error string.
799 You can assign a number to C<$!> to set I<errno> if, for instance,
800 you want C<"$!"> to return the string for error I<n>, or you want
801 to set the exit value for the die() operator. (Mnemonic: What just
804 Also see L<Error Indicators>.
813 Each element of C<%!> has a true value only if C<$!> is set to that
814 value. For example, C<$!{ENOENT}> is true if and only if the current
815 value of C<$!> is C<ENOENT>; that is, if the most recent error was
816 "No such file or directory" (or its moral equivalent: not all operating
817 systems give that exact error, and certainly not all languages).
818 To check if a particular key is meaningful on your system, use
819 C<exists $!{the_key}>; for a list of legal keys, use C<keys %!>.
820 See L<Errno> for more information, and also see above for the
823 =item $EXTENDED_OS_ERROR
826 X<$^E> X<$EXTENDED_OS_ERROR>
828 Error information specific to the current operating system. At
829 the moment, this differs from C<$!> under only VMS, OS/2, and Win32
830 (and for MacPerl). On all other platforms, C<$^E> is always just
833 Under VMS, C<$^E> provides the VMS status value from the last
834 system error. This is more specific information about the last
835 system error than that provided by C<$!>. This is particularly
836 important when C<$!> is set to B<EVMSERR>.
838 Under OS/2, C<$^E> is set to the error code of the last call to
839 OS/2 API either via CRT, or directly from perl.
841 Under Win32, C<$^E> always returns the last error information
842 reported by the Win32 call C<GetLastError()> which describes
843 the last error from within the Win32 API. Most Win32-specific
844 code will report errors via C<$^E>. ANSI C and Unix-like calls
845 set C<errno> and so most portable Perl code will report errors
848 Caveats mentioned in the description of C<$!> generally apply to
849 C<$^E>, also. (Mnemonic: Extra error explanation.)
851 Also see L<Error Indicators>.
858 The Perl syntax error message from the last eval() operator.
859 If $@ is the null string, the last eval() parsed and executed
860 correctly (although the operations you invoked may have failed in the
861 normal fashion). (Mnemonic: Where was the syntax error "at"?)
863 Warning messages are not collected in this variable. You can,
864 however, set up a routine to process warnings by setting C<$SIG{__WARN__}>
867 Also see L<Error Indicators>.
874 X<$$> X<$PID> X<$PROCESS_ID>
876 The process number of the Perl running this script. You should
877 consider this variable read-only, although it will be altered
878 across fork() calls. (Mnemonic: same as shells.)
880 Note for Linux users: on Linux, the C functions C<getpid()> and
881 C<getppid()> return different values from different threads. In order to
882 be portable, this behavior is not reflected by C<$$>, whose value remains
883 consistent across threads. If you want to call the underlying C<getpid()>,
884 you may use the CPAN module C<Linux::Pid>.
891 X<< $< >> X<$UID> X<$REAL_USER_ID>
893 The real uid of this process. (Mnemonic: it's the uid you came I<from>,
894 if you're running setuid.) You can change both the real uid and
895 the effective uid at the same time by using POSIX::setuid(). Since
896 changes to $< require a system call, check $! after a change attempt to
897 detect any possible errors.
899 =item $EFFECTIVE_USER_ID
904 X<< $> >> X<$EUID> X<$EFFECTIVE_USER_ID>
906 The effective uid of this process. Example:
908 $< = $>; # set real to effective uid
909 ($<,$>) = ($>,$<); # swap real and effective uid
911 You can change both the effective uid and the real uid at the same
912 time by using POSIX::setuid(). Changes to $> require a check to $!
913 to detect any possible errors after an attempted change.
915 (Mnemonic: it's the uid you went I<to>, if you're running setuid.)
916 C<< $< >> and C<< $> >> can be swapped only on machines
917 supporting setreuid().
924 X<$(> X<$GID> X<$REAL_GROUP_ID>
926 The real gid of this process. If you are on a machine that supports
927 membership in multiple groups simultaneously, gives a space separated
928 list of groups you are in. The first number is the one returned by
929 getgid(), and the subsequent ones by getgroups(), one of which may be
930 the same as the first number.
932 However, a value assigned to C<$(> must be a single number used to
933 set the real gid. So the value given by C<$(> should I<not> be assigned
934 back to C<$(> without being forced numeric, such as by adding zero. Note
935 that this is different to the effective gid (C<$)>) which does take a
938 You can change both the real gid and the effective gid at the same
939 time by using POSIX::setgid(). Changes to $( require a check to $!
940 to detect any possible errors after an attempted change.
942 (Mnemonic: parentheses are used to I<group> things. The real gid is the
943 group you I<left>, if you're running setgid.)
945 =item $EFFECTIVE_GROUP_ID
950 X<$)> X<$EGID> X<$EFFECTIVE_GROUP_ID>
952 The effective gid of this process. If you are on a machine that
953 supports membership in multiple groups simultaneously, gives a space
954 separated list of groups you are in. The first number is the one
955 returned by getegid(), and the subsequent ones by getgroups(), one of
956 which may be the same as the first number.
958 Similarly, a value assigned to C<$)> must also be a space-separated
959 list of numbers. The first number sets the effective gid, and
960 the rest (if any) are passed to setgroups(). To get the effect of an
961 empty list for setgroups(), just repeat the new effective gid; that is,
962 to force an effective gid of 5 and an effectively empty setgroups()
963 list, say C< $) = "5 5" >.
965 You can change both the effective gid and the real gid at the same
966 time by using POSIX::setgid() (use only a single numeric argument).
967 Changes to $) require a check to $! to detect any possible errors
968 after an attempted change.
970 (Mnemonic: parentheses are used to I<group> things. The effective gid
971 is the group that's I<right> for you, if you're running setgid.)
973 C<< $< >>, C<< $> >>, C<$(> and C<$)> can be set only on
974 machines that support the corresponding I<set[re][ug]id()> routine. C<$(>
975 and C<$)> can be swapped only on machines supporting setregid().
980 X<$0> X<$PROGRAM_NAME>
982 Contains the name of the program being executed.
984 On some (read: not all) operating systems assigning to C<$0> modifies
985 the argument area that the C<ps> program sees. On some platforms you
986 may have to use special C<ps> options or a different C<ps> to see the
987 changes. Modifying the $0 is more useful as a way of indicating the
988 current program state than it is for hiding the program you're
989 running. (Mnemonic: same as B<sh> and B<ksh>.)
991 Note that there are platform specific limitations on the maximum
992 length of C<$0>. In the most extreme case it may be limited to the
993 space occupied by the original C<$0>.
995 In some platforms there may be arbitrary amount of padding, for
996 example space characters, after the modified name as shown by C<ps>.
997 In some platforms this padding may extend all the way to the original
998 length of the argument area, no matter what you do (this is the case
999 for example with Linux 2.2).
1001 Note for BSD users: setting C<$0> does not completely remove "perl"
1002 from the ps(1) output. For example, setting C<$0> to C<"foobar"> may
1003 result in C<"perl: foobar (perl)"> (whether both the C<"perl: "> prefix
1004 and the " (perl)" suffix are shown depends on your exact BSD variant
1005 and version). This is an operating system feature, Perl cannot help it.
1007 In multithreaded scripts Perl coordinates the threads so that any
1008 thread may modify its copy of the C<$0> and the change becomes visible
1009 to ps(1) (assuming the operating system plays along). Note that
1010 the view of C<$0> the other threads have will not change since they
1011 have their own copies of it.
1016 The index of the first element in an array, and of the first character
1017 in a substring. Default is 0, but you could theoretically set it
1018 to 1 to make Perl behave more like B<awk> (or Fortran) when
1019 subscripting and when evaluating the index() and substr() functions.
1020 (Mnemonic: [ begins subscripts.)
1022 As of release 5 of Perl, assignment to C<$[> is treated as a compiler
1023 directive, and cannot influence the behavior of any other file.
1024 (That's why you can only assign compile-time constants to it.)
1025 Its use is highly discouraged.
1027 Note that, unlike other compile-time directives (such as L<strict>),
1028 assignment to C<$[> can be seen from outer lexical scopes in the same file.
1029 However, you can use local() on it to strictly bind its value to a
1035 The version + patchlevel / 1000 of the Perl interpreter. This variable
1036 can be used to determine whether the Perl interpreter executing a
1037 script is in the right range of versions. (Mnemonic: Is this version
1038 of perl in the right bracket?) Example:
1040 warn "No checksumming!\n" if $] < 3.019;
1042 See also the documentation of C<use VERSION> and C<require VERSION>
1043 for a convenient way to fail if the running Perl interpreter is too old.
1045 The floating point representation can sometimes lead to inaccurate
1046 numeric comparisons. See C<$^V> for a more modern representation of
1047 the Perl version that allows accurate string comparisons.
1052 X<$^C> X<$COMPILING>
1054 The current value of the flag associated with the B<-c> switch.
1055 Mainly of use with B<-MO=...> to allow code to alter its behavior
1056 when being compiled, such as for example to AUTOLOAD at compile
1057 time rather than normal, deferred loading. Setting
1058 C<$^C = 1> is similar to calling C<B::minus_c>.
1063 X<$^D> X<$DEBUGGING>
1065 The current value of the debugging flags. (Mnemonic: value of B<-D>
1066 switch.) May be read or set. Like its command-line equivalent, you can use
1067 numeric or symbolic values, eg C<$^D = 10> or C<$^D = "st">.
1069 =item ${^RE_DEBUG_FLAGS}
1071 The current value of the regex debugging flags. Set to 0 for no debug output
1072 even when the re 'debug' module is loaded. See L<re> for details.
1074 =item ${^RE_TRIE_MAXBUF}
1076 Controls how certain regex optimisations are applied and how much memory they
1077 utilize. This value by default is 65536 which corresponds to a 512kB temporary
1078 cache. Set this to a higher value to trade memory for speed when matching
1079 large alternations. Set it to a lower value if you want the optimisations to
1080 be as conservative of memory as possible but still occur, and set it to a
1081 negative value to prevent the optimisation and conserve the most memory.
1082 Under normal situations this variable should be of no interest to you.
1084 =item $SYSTEM_FD_MAX
1087 X<$^F> X<$SYSTEM_FD_MAX>
1089 The maximum system file descriptor, ordinarily 2. System file
1090 descriptors are passed to exec()ed processes, while higher file
1091 descriptors are not. Also, during an open(), system file descriptors are
1092 preserved even if the open() fails. (Ordinary file descriptors are
1093 closed before the open() is attempted.) The close-on-exec
1094 status of a file descriptor will be decided according to the value of
1095 C<$^F> when the corresponding file, pipe, or socket was opened, not the
1100 WARNING: This variable is strictly for internal use only. Its availability,
1101 behavior, and contents are subject to change without notice.
1103 This variable contains compile-time hints for the Perl interpreter. At the
1104 end of compilation of a BLOCK the value of this variable is restored to the
1105 value when the interpreter started to compile the BLOCK.
1107 When perl begins to parse any block construct that provides a lexical scope
1108 (e.g., eval body, required file, subroutine body, loop body, or conditional
1109 block), the existing value of $^H is saved, but its value is left unchanged.
1110 When the compilation of the block is completed, it regains the saved value.
1111 Between the points where its value is saved and restored, code that
1112 executes within BEGIN blocks is free to change the value of $^H.
1114 This behavior provides the semantic of lexical scoping, and is used in,
1115 for instance, the C<use strict> pragma.
1117 The contents should be an integer; different bits of it are used for
1118 different pragmatic flags. Here's an example:
1120 sub add_100 { $^H |= 0x100 }
1127 Consider what happens during execution of the BEGIN block. At this point
1128 the BEGIN block has already been compiled, but the body of foo() is still
1129 being compiled. The new value of $^H will therefore be visible only while
1130 the body of foo() is being compiled.
1132 Substitution of the above BEGIN block with:
1134 BEGIN { require strict; strict->import('vars') }
1136 demonstrates how C<use strict 'vars'> is implemented. Here's a conditional
1137 version of the same lexical pragma:
1139 BEGIN { require strict; strict->import('vars') if $condition }
1143 The %^H hash provides the same scoping semantic as $^H. This makes it
1144 useful for implementation of lexically scoped pragmas. See L<perlpragma>.
1149 X<$^I> X<$INPLACE_EDIT>
1151 The current value of the inplace-edit extension. Use C<undef> to disable
1152 inplace editing. (Mnemonic: value of B<-i> switch.)
1157 By default, running out of memory is an untrappable, fatal error.
1158 However, if suitably built, Perl can use the contents of C<$^M>
1159 as an emergency memory pool after die()ing. Suppose that your Perl
1160 were compiled with C<-DPERL_EMERGENCY_SBRK> and used Perl's malloc.
1163 $^M = 'a' x (1 << 16);
1165 would allocate a 64K buffer for use in an emergency. See the
1166 F<INSTALL> file in the Perl distribution for information on how to
1167 add custom C compilation flags when compiling perl. To discourage casual
1168 use of this advanced feature, there is no L<English|English> long name for
1176 The name of the operating system under which this copy of Perl was
1177 built, as determined during the configuration process. The value
1178 is identical to C<$Config{'osname'}>. See also L<Config> and the
1179 B<-V> command-line switch documented in L<perlrun>.
1181 In Windows platforms, $^O is not very helpful: since it is always
1182 C<MSWin32>, it doesn't tell the difference between
1183 95/98/ME/NT/2000/XP/CE/.NET. Use Win32::GetOSName() or
1184 Win32::GetOSVersion() (see L<Win32> and L<perlport>) to distinguish
1185 between the variants.
1189 An internal variable used by PerlIO. A string in two parts, separated
1190 by a C<\0> byte, the first part describes the input layers, the second
1191 part describes the output layers.
1198 The internal variable for debugging support. The meanings of the
1199 various bits are subject to change, but currently indicate:
1205 Debug subroutine enter/exit.
1209 Line-by-line debugging.
1213 Switch off optimizations.
1217 Preserve more data for future interactive inspections.
1221 Keep info about source lines on which a subroutine is defined.
1225 Start with single-step on.
1229 Use subroutine address instead of name when reporting.
1233 Report C<goto &subroutine> as well.
1237 Provide informative "file" names for evals based on the place they were compiled.
1241 Provide informative names to anonymous subroutines based on the place they
1246 Debug assertion subroutines enter/exit.
1250 Some bits may be relevant at compile-time only, some at
1251 run-time only. This is a new mechanism and the details may change.
1253 =item $LAST_REGEXP_CODE_RESULT
1256 X<$^R> X<$LAST_REGEXP_CODE_RESULT>
1258 The result of evaluation of the last successful C<(?{ code })>
1259 regular expression assertion (see L<perlre>). May be written to.
1261 =item $EXCEPTIONS_BEING_CAUGHT
1264 X<$^S> X<$EXCEPTIONS_BEING_CAUGHT>
1266 Current state of the interpreter.
1269 --------- -------------------
1270 undef Parsing module/eval
1271 true (1) Executing an eval
1274 The first state may happen in $SIG{__DIE__} and $SIG{__WARN__} handlers.
1281 The time at which the program began running, in seconds since the
1282 epoch (beginning of 1970). The values returned by the B<-M>, B<-A>,
1283 and B<-C> filetests are based on this value.
1287 Reflects if taint mode is on or off. 1 for on (the program was run with
1288 B<-T>), 0 for off, -1 when only taint warnings are enabled (i.e. with
1289 B<-t> or B<-TU>). This variable is read-only.
1293 Reflects certain Unicode settings of Perl. See L<perlrun>
1294 documentation for the C<-C> switch for more information about
1295 the possible values. This variable is set during Perl startup
1296 and is thereafter read-only.
1300 This variable controls the state of the internal UTF-8 offset caching code.
1301 1 for on (the default), 0 for off, -1 to debug the caching code by checking
1302 all its results against linear scans, and panicking on any discrepancy.
1304 =item ${^UTF8LOCALE}
1306 This variable indicates whether an UTF-8 locale was detected by perl at
1307 startup. This information is used by perl when it's in
1308 adjust-utf8ness-to-locale mode (as when run with the C<-CL> command-line
1309 switch); see L<perlrun> for more info on this.
1314 X<$^V> X<$PERL_VERSION>
1316 The revision, version, and subversion of the Perl interpreter, represented
1317 as a string composed of characters with those ordinals. Thus in Perl v5.6.0
1318 it equals C<chr(5) . chr(6) . chr(0)> and will return true for
1319 C<$^V eq v5.6.0>. Note that the characters in this string value can
1320 potentially be greater than 255.
1322 This variable first appeared in perl 5.6.0; earlier versions of perl will
1323 see an undefined value.
1325 This can be used to determine whether the Perl interpreter executing a
1326 script is in the right range of versions. (Mnemonic: use ^V for Version
1329 warn "Hashes not randomized!\n" if !$^V or $^V lt v5.8.1
1331 To convert C<$^V> into its string representation use sprintf()'s
1332 C<"%vd"> conversion:
1334 printf "version is v%vd\n", $^V; # Perl's version
1336 See the documentation of C<use VERSION> and C<require VERSION>
1337 for a convenient way to fail if the running Perl interpreter is too old.
1339 See also C<$]> for an older representation of the Perl version.
1346 The current value of the warning switch, initially true if B<-w>
1347 was used, false otherwise, but directly modifiable. (Mnemonic:
1348 related to the B<-w> switch.) See also L<warnings>.
1350 =item ${^WARNING_BITS}
1352 The current set of warning checks enabled by the C<use warnings> pragma.
1353 See the documentation of C<warnings> for more details.
1355 =item ${^WIN32_SLOPPY_STAT}
1357 If this variable is set to a true value, then stat() on Windows will
1358 not try to open the file. This means that the link count cannot be
1359 determined and file attributes may be out of date if additional
1360 hardlinks to the file exist. On the other hand, not opening the file
1361 is considerably faster, especially for files on network drives.
1363 This variable could be set in the F<sitecustomize.pl> file to
1364 configure the local Perl installation to use "sloppy" stat() by
1365 default. See L<perlrun> for more information about site
1368 =item $EXECUTABLE_NAME
1371 X<$^X> X<$EXECUTABLE_NAME>
1373 The name used to execute the current copy of Perl, from C's
1374 C<argv[0]> or (where supported) F</proc/self/exe>.
1376 Depending on the host operating system, the value of $^X may be
1377 a relative or absolute pathname of the perl program file, or may
1378 be the string used to invoke perl but not the pathname of the
1379 perl program file. Also, most operating systems permit invoking
1380 programs that are not in the PATH environment variable, so there
1381 is no guarantee that the value of $^X is in PATH. For VMS, the
1382 value may or may not include a version number.
1384 You usually can use the value of $^X to re-invoke an independent
1385 copy of the same perl that is currently running, e.g.,
1387 @first_run = `$^X -le "print int rand 100 for 1..100"`;
1389 But recall that not all operating systems support forking or
1390 capturing of the output of commands, so this complex statement
1391 may not be portable.
1393 It is not safe to use the value of $^X as a path name of a file,
1394 as some operating systems that have a mandatory suffix on
1395 executable files do not require use of the suffix when invoking
1396 a command. To convert the value of $^X to a path name, use the
1397 following statements:
1399 # Build up a set of file names (not command names).
1403 {$this_perl .= $Config{_exe}
1404 unless $this_perl =~ m/$Config{_exe}$/i;}
1406 Because many operating systems permit anyone with read access to
1407 the Perl program file to make a copy of it, patch the copy, and
1408 then execute the copy, the security-conscious Perl programmer
1409 should take care to invoke the installed copy of perl, not the
1410 copy referenced by $^X. The following statements accomplish
1411 this goal, and produce a pathname that can be invoked as a
1412 command or referenced as a file.
1415 $secure_perl_path = $Config{perlpath};
1417 {$secure_perl_path .= $Config{_exe}
1418 unless $secure_perl_path =~ m/$Config{_exe}$/i;}
1423 The special filehandle that iterates over command-line filenames in
1424 C<@ARGV>. Usually written as the null filehandle in the angle operator
1425 C<< <> >>. Note that currently C<ARGV> only has its magical effect
1426 within the C<< <> >> operator; elsewhere it is just a plain filehandle
1427 corresponding to the last file opened by C<< <> >>. In particular,
1428 passing C<\*ARGV> as a parameter to a function that expects a filehandle
1429 may not cause your function to automatically read the contents of all the
1435 contains the name of the current file when reading from <>.
1440 The array @ARGV contains the command-line arguments intended for
1441 the script. C<$#ARGV> is generally the number of arguments minus
1442 one, because C<$ARGV[0]> is the first argument, I<not> the program's
1443 command name itself. See C<$0> for the command name.
1448 The special filehandle that points to the currently open output file
1449 when doing edit-in-place processing with B<-i>. Useful when you have
1450 to do a lot of inserting and don't want to keep modifying $_. See
1451 L<perlrun> for the B<-i> switch.
1456 The array @F contains the fields of each line read in when autosplit
1457 mode is turned on. See L<perlrun> for the B<-a> switch. This array
1458 is package-specific, and must be declared or given a full package name
1459 if not in package main when running under C<strict 'vars'>.
1464 The array @INC contains the list of places that the C<do EXPR>,
1465 C<require>, or C<use> constructs look for their library files. It
1466 initially consists of the arguments to any B<-I> command-line
1467 switches, followed by the default Perl library, probably
1468 F</usr/local/lib/perl>, followed by ".", to represent the current
1469 directory. ("." will not be appended if taint checks are enabled, either by
1470 C<-T> or by C<-t>.) If you need to modify this at runtime, you should use
1471 the C<use lib> pragma to get the machine-dependent library properly
1474 use lib '/mypath/libdir/';
1477 You can also insert hooks into the file inclusion system by putting Perl
1478 code directly into @INC. Those hooks may be subroutine references, array
1479 references or blessed objects. See L<perlfunc/require> for details.
1486 Within a subroutine the array @_ contains the parameters passed to that
1487 subroutine. See L<perlsub>.
1492 The hash %INC contains entries for each filename included via the
1493 C<do>, C<require>, or C<use> operators. The key is the filename
1494 you specified (with module names converted to pathnames), and the
1495 value is the location of the file found. The C<require>
1496 operator uses this hash to determine whether a particular file has
1497 already been included.
1499 If the file was loaded via a hook (e.g. a subroutine reference, see
1500 L<perlfunc/require> for a description of these hooks), this hook is
1501 by default inserted into %INC in place of a filename. Note, however,
1502 that the hook may have set the %INC entry by itself to provide some more
1510 The hash %ENV contains your current environment. Setting a
1511 value in C<ENV> changes the environment for any child processes
1512 you subsequently fork() off.
1519 The hash C<%SIG> contains signal handlers for signals. For example:
1521 sub handler { # 1st argument is signal name
1523 print "Caught a SIG$sig--shutting down\n";
1528 $SIG{'INT'} = \&handler;
1529 $SIG{'QUIT'} = \&handler;
1531 $SIG{'INT'} = 'DEFAULT'; # restore default action
1532 $SIG{'QUIT'} = 'IGNORE'; # ignore SIGQUIT
1534 Using a value of C<'IGNORE'> usually has the effect of ignoring the
1535 signal, except for the C<CHLD> signal. See L<perlipc> for more about
1538 Here are some other examples:
1540 $SIG{"PIPE"} = "Plumber"; # assumes main::Plumber (not recommended)
1541 $SIG{"PIPE"} = \&Plumber; # just fine; assume current Plumber
1542 $SIG{"PIPE"} = *Plumber; # somewhat esoteric
1543 $SIG{"PIPE"} = Plumber(); # oops, what did Plumber() return??
1545 Be sure not to use a bareword as the name of a signal handler,
1546 lest you inadvertently call it.
1548 If your system has the sigaction() function then signal handlers are
1549 installed using it. This means you get reliable signal handling.
1551 The default delivery policy of signals changed in Perl 5.8.0 from
1552 immediate (also known as "unsafe") to deferred, also known as
1553 "safe signals". See L<perlipc> for more information.
1555 Certain internal hooks can be also set using the %SIG hash. The
1556 routine indicated by C<$SIG{__WARN__}> is called when a warning message is
1557 about to be printed. The warning message is passed as the first
1558 argument. The presence of a C<__WARN__> hook causes the ordinary printing
1559 of warnings to C<STDERR> to be suppressed. You can use this to save warnings
1560 in a variable, or turn warnings into fatal errors, like this:
1562 local $SIG{__WARN__} = sub { die $_[0] };
1565 As the C<'IGNORE'> hook is not supported by C<__WARN__>, you can
1566 disable warnings using the empty subroutine:
1568 local $SIG{__WARN__} = sub {};
1570 The routine indicated by C<$SIG{__DIE__}> is called when a fatal exception
1571 is about to be thrown. The error message is passed as the first
1572 argument. When a C<__DIE__> hook routine returns, the exception
1573 processing continues as it would have in the absence of the hook,
1574 unless the hook routine itself exits via a C<goto>, a loop exit, or a C<die()>.
1575 The C<__DIE__> handler is explicitly disabled during the call, so that you
1576 can die from a C<__DIE__> handler. Similarly for C<__WARN__>.
1578 Due to an implementation glitch, the C<$SIG{__DIE__}> hook is called
1579 even inside an eval(). Do not use this to rewrite a pending exception
1580 in C<$@>, or as a bizarre substitute for overriding C<CORE::GLOBAL::die()>.
1581 This strange action at a distance may be fixed in a future release
1582 so that C<$SIG{__DIE__}> is only called if your program is about
1583 to exit, as was the original intent. Any other use is deprecated.
1585 C<__DIE__>/C<__WARN__> handlers are very special in one respect:
1586 they may be called to report (probable) errors found by the parser.
1587 In such a case the parser may be in inconsistent state, so any
1588 attempt to evaluate Perl code from such a handler will probably
1589 result in a segfault. This means that warnings or errors that
1590 result from parsing Perl should be used with extreme caution, like
1593 require Carp if defined $^S;
1594 Carp::confess("Something wrong") if defined &Carp::confess;
1595 die "Something wrong, but could not load Carp to give backtrace...
1596 To see backtrace try starting Perl with -MCarp switch";
1598 Here the first line will load Carp I<unless> it is the parser who
1599 called the handler. The second line will print backtrace and die if
1600 Carp was available. The third line will be executed only if Carp was
1603 See L<perlfunc/die>, L<perlfunc/warn>, L<perlfunc/eval>, and
1604 L<warnings> for additional information.
1608 =head2 Error Indicators
1609 X<error> X<exception>
1611 The variables C<$@>, C<$!>, C<$^E>, and C<$?> contain information
1612 about different types of error conditions that may appear during
1613 execution of a Perl program. The variables are shown ordered by
1614 the "distance" between the subsystem which reported the error and
1615 the Perl process. They correspond to errors detected by the Perl
1616 interpreter, C library, operating system, or an external program,
1619 To illustrate the differences between these variables, consider the
1620 following Perl expression, which uses a single-quoted string:
1623 open my $pipe, "/cdrom/install |" or die $!;
1625 close $pipe or die "bad pipe: $?, $!";
1628 After execution of this statement all 4 variables may have been set.
1630 C<$@> is set if the string to be C<eval>-ed did not compile (this
1631 may happen if C<open> or C<close> were imported with bad prototypes),
1632 or if Perl code executed during evaluation die()d . In these cases
1633 the value of $@ is the compile error, or the argument to C<die>
1634 (which will interpolate C<$!> and C<$?>). (See also L<Fatal>,
1637 When the eval() expression above is executed, open(), C<< <PIPE> >>,
1638 and C<close> are translated to calls in the C run-time library and
1639 thence to the operating system kernel. C<$!> is set to the C library's
1640 C<errno> if one of these calls fails.
1642 Under a few operating systems, C<$^E> may contain a more verbose
1643 error indicator, such as in this case, "CDROM tray not closed."
1644 Systems that do not support extended error messages leave C<$^E>
1647 Finally, C<$?> may be set to non-0 value if the external program
1648 F</cdrom/install> fails. The upper eight bits reflect specific
1649 error conditions encountered by the program (the program's exit()
1650 value). The lower eight bits reflect mode of failure, like signal
1651 death and core dump information See wait(2) for details. In
1652 contrast to C<$!> and C<$^E>, which are set only if error condition
1653 is detected, the variable C<$?> is set on each C<wait> or pipe
1654 C<close>, overwriting the old value. This is more like C<$@>, which
1655 on every eval() is always set on failure and cleared on success.
1657 For more details, see the individual descriptions at C<$@>, C<$!>, C<$^E>,
1660 =head2 Technical Note on the Syntax of Variable Names
1662 Variable names in Perl can have several formats. Usually, they
1663 must begin with a letter or underscore, in which case they can be
1664 arbitrarily long (up to an internal limit of 251 characters) and
1665 may contain letters, digits, underscores, or the special sequence
1666 C<::> or C<'>. In this case, the part before the last C<::> or
1667 C<'> is taken to be a I<package qualifier>; see L<perlmod>.
1669 Perl variable names may also be a sequence of digits or a single
1670 punctuation or control character. These names are all reserved for
1671 special uses by Perl; for example, the all-digits names are used
1672 to hold data captured by backreferences after a regular expression
1673 match. Perl has a special syntax for the single-control-character
1674 names: It understands C<^X> (caret C<X>) to mean the control-C<X>
1675 character. For example, the notation C<$^W> (dollar-sign caret
1676 C<W>) is the scalar variable whose name is the single character
1677 control-C<W>. This is better than typing a literal control-C<W>
1680 Finally, new in Perl 5.6, Perl variable names may be alphanumeric
1681 strings that begin with control characters (or better yet, a caret).
1682 These variables must be written in the form C<${^Foo}>; the braces
1683 are not optional. C<${^Foo}> denotes the scalar variable whose
1684 name is a control-C<F> followed by two C<o>'s. These variables are
1685 reserved for future special uses by Perl, except for the ones that
1686 begin with C<^_> (control-underscore or caret-underscore). No
1687 control-character name that begins with C<^_> will acquire a special
1688 meaning in any future version of Perl; such names may therefore be
1689 used safely in programs. C<$^_> itself, however, I<is> reserved.
1691 Perl identifiers that begin with digits, control characters, or
1692 punctuation characters are exempt from the effects of the C<package>
1693 declaration and are always forced to be in package C<main>; they are
1694 also exempt from C<strict 'vars'> errors. A few other names are also
1695 exempt in these ways:
1703 In particular, the new special C<${^_XYZ}> variables are always taken
1704 to be in package C<main>, regardless of any C<package> declarations
1709 Due to an unfortunate accident of Perl's implementation, C<use
1710 English> imposes a considerable performance penalty on all regular
1711 expression matches in a program, regardless of whether they occur
1712 in the scope of C<use English>. For that reason, saying C<use
1713 English> in libraries is strongly discouraged. See the
1714 Devel::SawAmpersand module documentation from CPAN
1715 ( http://www.cpan.org/modules/by-module/Devel/ )
1716 for more information. Writing C<use English '-no_match_vars';>
1717 avoids the performance penalty.
1719 Having to even think about the C<$^S> variable in your exception
1720 handlers is simply wrong. C<$SIG{__DIE__}> as currently implemented
1721 invites grievous and difficult to track down errors. Avoid it
1722 and use an C<END{}> or CORE::GLOBAL::die override instead.