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 will alias all the short names to the
17 long names in the current package. Some even have medium names,
18 generally borrowed from B<awk>.
20 If you don't mind the performance hit, variables that depend on the
21 currently selected filehandle may instead be set by calling an
22 appropriate object method on the IO::Handle object. (Summary lines
23 below for this contain the word HANDLE.) First you must say
27 after which you may use either
35 Each method returns the old value of the IO::Handle attribute.
36 The methods each take an optional EXPR, which if supplied specifies the
37 new value for the IO::Handle attribute in question. If not supplied,
38 most methods do nothing to the current value--except for
39 autoflush(), which will assume a 1 for you, just to be different.
40 Because loading in the IO::Handle class is an expensive operation, you should
41 learn how to use the regular built-in variables.
43 A few of these variables are considered "read-only". This means that if
44 you try to assign to this variable, either directly or indirectly through
45 a reference, you'll raise a run-time exception.
47 The following list is ordered by scalar variables first, then the
48 arrays, then the hashes.
56 The default input and pattern-searching space. The following pairs are
59 while (<>) {...} # equivalent only in while!
60 while (defined($_ = <>)) {...}
71 Here are the places where Perl will assume $_ even if you
78 Various unary functions, including functions like ord() and int(), as well
79 as the all file tests (C<-f>, C<-d>) except for C<-t>, which defaults to
84 Various list functions like print() and unlink().
88 The pattern matching operations C<m//>, C<s///>, and C<tr///> when used
89 without an C<=~> operator.
93 The default iterator variable in a C<foreach> loop if no other
98 The implicit iterator variable in the grep() and map() functions.
102 The default place to put an input record when a C<< <FH> >>
103 operation's result is tested by itself as the sole criterion of a C<while>
104 test. Outside a C<while> test, this will not happen.
108 (Mnemonic: underline is understood in certain operations.)
118 Special package variables when using sort(), see L<perlfunc/sort>.
119 Because of this specialness $a and $b don't need to be declared
120 (using local(), use vars, or our()) even when using the strict
121 vars pragma. Don't lexicalize them with C<my $a> or C<my $b>
122 if you want to be able to use them in the sort() comparison block
131 Contains the subpattern from the corresponding set of capturing
132 parentheses from the last pattern match, not counting patterns
133 matched in nested blocks that have been exited already. (Mnemonic:
134 like \digits.) These variables are all read-only and dynamically
135 scoped to the current BLOCK.
141 The string matched by the last successful pattern match (not counting
142 any matches hidden within a BLOCK or eval() enclosed by the current
143 BLOCK). (Mnemonic: like & in some editors.) This variable is read-only
144 and dynamically scoped to the current BLOCK.
146 The use of this variable anywhere in a program imposes a considerable
147 performance penalty on all regular expression matches. See L<BUGS>.
153 The string preceding whatever was matched by the last successful
154 pattern match (not counting any matches hidden within a BLOCK or eval
155 enclosed by the current BLOCK). (Mnemonic: C<`> often precedes a quoted
156 string.) This variable is read-only.
158 The use of this variable anywhere in a program imposes a considerable
159 performance penalty on all regular expression matches. See L<BUGS>.
165 The string following whatever was matched by the last successful
166 pattern match (not counting any matches hidden within a BLOCK or eval()
167 enclosed by the current BLOCK). (Mnemonic: C<'> often follows a quoted
172 print "$`:$&:$'\n"; # prints abc:def:ghi
174 This variable is read-only and dynamically scoped to the current BLOCK.
176 The use of this variable anywhere in a program imposes a considerable
177 performance penalty on all regular expression matches. See L<BUGS>.
179 =item $LAST_PAREN_MATCH
183 The text matched by the last bracket of the last successful search pattern.
184 This is useful if you don't know which one of a set of alternative patterns
185 matched. For example:
187 /Version: (.*)|Revision: (.*)/ && ($rev = $+);
189 (Mnemonic: be positive and forward looking.)
190 This variable is read-only and dynamically scoped to the current BLOCK.
194 The text matched by the used group most-recently closed (i.e. the group
195 with the rightmost closing parenthesis) of the last successful search
196 pattern. (Mnemonic: the (possibly) Nested parenthesis that most
199 This is primarly used inside C<(?{...})> blocks for examining text
200 recently matched. For example, to effectively capture text to a variable
201 (in addition to C<$1>, C<$2>, etc.), replace C<(...)> with
203 (?:(...)(?{ $var = $^N }))
205 By setting and then using C<$var> in this way relieves you from having to
206 worry about exactly which numbered set of parentheses they are.
208 This variable is dynamically scoped to the current BLOCK.
210 =item @LAST_MATCH_END
214 This array holds the offsets of the ends of the last successful
215 submatches in the currently active dynamic scope. C<$+[0]> is
216 the offset into the string of the end of the entire match. This
217 is the same value as what the C<pos> function returns when called
218 on the variable that was matched against. The I<n>th element
219 of this array holds the offset of the I<n>th submatch, so
220 C<$+[1]> is the offset past where $1 ends, C<$+[2]> the offset
221 past where $2 ends, and so on. You can use C<$#+> to determine
222 how many subgroups were in the last successful match. See the
223 examples given for the C<@-> variable.
225 =item $MULTILINE_MATCHING
229 Set to a non-zero integer value to do multi-line matching within a
230 string, 0 (or undefined) to tell Perl that it can assume that strings
231 contain a single line, for the purpose of optimizing pattern matches.
232 Pattern matches on strings containing multiple newlines can produce
233 confusing results when C<$*> is 0 or undefined. Default is undefined.
234 (Mnemonic: * matches multiple things.) This variable influences the
235 interpretation of only C<^> and C<$>. A literal newline can be searched
236 for even when C<$* == 0>.
238 Use of C<$*> is deprecated in modern Perl, supplanted by
239 the C</s> and C</m> modifiers on pattern matching.
241 Assigning a non-numerical value to C<$*> triggers a warning (and makes
242 C<$*> act if C<$* == 0>), while assigning a numerical value to C<$*>
243 makes that an implicit C<int> is applied on the value.
245 =item input_line_number HANDLE EXPR
247 =item $INPUT_LINE_NUMBER
253 The current input record number for the last file handle from which
254 you just read() (or called a C<seek> or C<tell> on). The value
255 may be different from the actual physical line number in the file,
256 depending on what notion of "line" is in effect--see C<$/> on how
257 to change that. An explicit close on a filehandle resets the line
258 number. Because C<< <> >> never does an explicit close, line
259 numbers increase across ARGV files (but see examples in L<perlfunc/eof>).
260 Consider this variable read-only: setting it does not reposition
261 the seek pointer; you'll have to do that on your own. Localizing C<$.>
262 has the effect of also localizing Perl's notion of "the last read
263 filehandle". (Mnemonic: many programs use "." to mean the current line
266 =item input_record_separator HANDLE EXPR
268 =item $INPUT_RECORD_SEPARATOR
274 The input record separator, newline by default. This
275 influences Perl's idea of what a "line" is. Works like B<awk>'s RS
276 variable, including treating empty lines as a terminator if set to
277 the null string. (An empty line cannot contain any spaces
278 or tabs.) You may set it to a multi-character string to match a
279 multi-character terminator, or to C<undef> to read through the end
280 of file. Setting it to C<"\n\n"> means something slightly
281 different than setting to C<"">, if the file contains consecutive
282 empty lines. Setting to C<""> will treat two or more consecutive
283 empty lines as a single empty line. Setting to C<"\n\n"> will
284 blindly assume that the next input character belongs to the next
285 paragraph, even if it's a newline. (Mnemonic: / delimits
286 line boundaries when quoting poetry.)
288 undef $/; # enable "slurp" mode
289 $_ = <FH>; # whole file now here
292 Remember: the value of C<$/> is a string, not a regex. B<awk> has to be
293 better for something. :-)
295 Setting C<$/> to a reference to an integer, scalar containing an integer, or
296 scalar that's convertible to an integer will attempt to read records
297 instead of lines, with the maximum record size being the referenced
300 $/ = \32768; # or \"32768", or \$var_containing_32768
304 will read a record of no more than 32768 bytes from FILE. If you're
305 not reading from a record-oriented file (or your OS doesn't have
306 record-oriented files), then you'll likely get a full chunk of data
307 with every read. If a record is larger than the record size you've
308 set, you'll get the record back in pieces.
310 On VMS, record reads are done with the equivalent of C<sysread>,
311 so it's best not to mix record and non-record reads on the same
312 file. (This is unlikely to be a problem, because any file you'd
313 want to read in record mode is probably unusable in line mode.)
314 Non-VMS systems do normal I/O, so it's safe to mix record and
315 non-record reads of a file.
317 See also L<perlport/"Newlines">. Also see C<$.>.
319 =item autoflush HANDLE EXPR
321 =item $OUTPUT_AUTOFLUSH
325 If set to nonzero, forces a flush right away and after every write
326 or print on the currently selected output channel. Default is 0
327 (regardless of whether the channel is really buffered by the
328 system or not; C<$|> tells you only whether you've asked Perl
329 explicitly to flush after each write). STDOUT will
330 typically be line buffered if output is to the terminal and block
331 buffered otherwise. Setting this variable is useful primarily when
332 you are outputting to a pipe or socket, such as when you are running
333 a Perl program under B<rsh> and want to see the output as it's
334 happening. This has no effect on input buffering. See L<perlfunc/getc>
335 for that. (Mnemonic: when you want your pipes to be piping hot.)
337 =item output_field_separator HANDLE EXPR
339 =item $OUTPUT_FIELD_SEPARATOR
345 The output field separator for the print operator. Ordinarily the
346 print operator simply prints out its arguments without further
347 adornment. To get behavior more like B<awk>, set this variable as
348 you would set B<awk>'s OFS variable to specify what is printed
349 between fields. (Mnemonic: what is printed when there is a "," in
350 your print statement.)
352 =item output_record_separator HANDLE EXPR
354 =item $OUTPUT_RECORD_SEPARATOR
360 The output record separator for the print operator. Ordinarily the
361 print operator simply prints out its arguments as is, with no
362 trailing newline or other end-of-record string added. To get
363 behavior more like B<awk>, set this variable as you would set
364 B<awk>'s ORS variable to specify what is printed at the end of the
365 print. (Mnemonic: you set C<$\> instead of adding "\n" at the
366 end of the print. Also, it's just like C<$/>, but it's what you
367 get "back" from Perl.)
369 =item $LIST_SEPARATOR
373 This is like C<$,> except that it applies to array and slice values
374 interpolated into a double-quoted string (or similar interpreted
375 string). Default is a space. (Mnemonic: obvious, I think.)
377 =item $SUBSCRIPT_SEPARATOR
383 The subscript separator for multidimensional array emulation. If you
384 refer to a hash element as
390 $foo{join($;, $a, $b, $c)}
394 @foo{$a,$b,$c} # a slice--note the @
398 ($foo{$a},$foo{$b},$foo{$c})
400 Default is "\034", the same as SUBSEP in B<awk>. If your
401 keys contain binary data there might not be any safe value for C<$;>.
402 (Mnemonic: comma (the syntactic subscript separator) is a
403 semi-semicolon. Yeah, I know, it's pretty lame, but C<$,> is already
404 taken for something more important.)
406 Consider using "real" multidimensional arrays as described
413 The output format for printed numbers. This variable is a half-hearted
414 attempt to emulate B<awk>'s OFMT variable. There are times, however,
415 when B<awk> and Perl have differing notions of what counts as
416 numeric. The initial value is "%.I<n>g", where I<n> is the value
417 of the macro DBL_DIG from your system's F<float.h>. This is different from
418 B<awk>'s default OFMT setting of "%.6g", so you need to set C<$#>
419 explicitly to get B<awk>'s value. (Mnemonic: # is the number sign.)
421 Use of C<$#> is deprecated.
423 =item format_page_number HANDLE EXPR
425 =item $FORMAT_PAGE_NUMBER
429 The current page number of the currently selected output channel.
431 (Mnemonic: % is page number in B<nroff>.)
433 =item format_lines_per_page HANDLE EXPR
435 =item $FORMAT_LINES_PER_PAGE
439 The current page length (printable lines) of the currently selected
440 output channel. Default is 60.
442 (Mnemonic: = has horizontal lines.)
444 =item format_lines_left HANDLE EXPR
446 =item $FORMAT_LINES_LEFT
450 The number of lines left on the page of the currently selected output
453 (Mnemonic: lines_on_page - lines_printed.)
455 =item @LAST_MATCH_START
459 $-[0] is the offset of the start of the last successful match.
460 C<$-[>I<n>C<]> is the offset of the start of the substring matched by
461 I<n>-th subpattern, or undef if the subpattern did not match.
463 Thus after a match against $_, $& coincides with C<substr $_, $-[0],
464 $+[0] - $-[0]>. Similarly, C<$>I<n> coincides with C<substr $_, $-[>I<n>C<],
465 $+[>I<n>C<] - $-[>I<n>C<]> if C<$-[>I<n>C<]> is defined, and $+ coincides with
466 C<substr $_, $-[$#-], $+[$#-]>. One can use C<$#-> to find the last
467 matched subgroup in the last successful match. Contrast with
468 C<$#+>, the number of subgroups in the regular expression. Compare
471 This array holds the offsets of the beginnings of the last
472 successful submatches in the currently active dynamic scope.
473 C<$-[0]> is the offset into the string of the beginning of the
474 entire match. The I<n>th element of this array holds the offset
475 of the I<n>th submatch, so C<$+[1]> is the offset where $1
476 begins, C<$+[2]> the offset where $2 begins, and so on.
477 You can use C<$#-> to determine how many subgroups were in the
478 last successful match. Compare with the C<@+> variable.
480 After a match against some variable $var:
484 =item C<$`> is the same as C<substr($var, 0, $-[0])>
486 =item C<$&> is the same as C<substr($var, $-[0], $+[0] - $-[0])>
488 =item C<$'> is the same as C<substr($var, $+[0])>
490 =item C<$1> is the same as C<substr($var, $-[1], $+[1] - $-[1])>
492 =item C<$2> is the same as C<substr($var, $-[2], $+[2] - $-[2])>
494 =item C<$3> is the same as C<substr $var, $-[3], $+[3] - $-[3])>
498 =item format_name HANDLE EXPR
504 The name of the current report format for the currently selected output
505 channel. Default is the name of the filehandle. (Mnemonic: brother to
508 =item format_top_name HANDLE EXPR
510 =item $FORMAT_TOP_NAME
514 The name of the current top-of-page format for the currently selected
515 output channel. Default is the name of the filehandle with _TOP
516 appended. (Mnemonic: points to top of page.)
518 =item format_line_break_characters HANDLE EXPR
520 =item $FORMAT_LINE_BREAK_CHARACTERS
524 The current set of characters after which a string may be broken to
525 fill continuation fields (starting with ^) in a format. Default is
526 S<" \n-">, to break on whitespace or hyphens. (Mnemonic: a "colon" in
527 poetry is a part of a line.)
529 =item format_formfeed HANDLE EXPR
531 =item $FORMAT_FORMFEED
535 What formats output as a form feed. Default is \f.
541 The current value of the write() accumulator for format() lines. A format
542 contains formline() calls that put their result into C<$^A>. After
543 calling its format, write() prints out the contents of C<$^A> and empties.
544 So you never really see the contents of C<$^A> unless you call
545 formline() yourself and then look at it. See L<perlform> and
546 L<perlfunc/formline()>.
552 The status returned by the last pipe close, backtick (C<``>) command,
553 successful call to wait() or waitpid(), or from the system()
554 operator. This is just the 16-bit status word returned by the
555 wait() system call (or else is made up to look like it). Thus, the
556 exit value of the subprocess is really (C<<< $? >> 8 >>>), and
557 C<$? & 127> gives which signal, if any, the process died from, and
558 C<$? & 128> reports whether there was a core dump. (Mnemonic:
559 similar to B<sh> and B<ksh>.)
561 Additionally, if the C<h_errno> variable is supported in C, its value
562 is returned via $? if any C<gethost*()> function fails.
564 If you have installed a signal handler for C<SIGCHLD>, the
565 value of C<$?> will usually be wrong outside that handler.
567 Inside an C<END> subroutine C<$?> contains the value that is going to be
568 given to C<exit()>. You can modify C<$?> in an C<END> subroutine to
569 change the exit status of your program. For example:
572 $? = 1 if $? == 255; # die would make it 255
575 Under VMS, the pragma C<use vmsish 'status'> makes C<$?> reflect the
576 actual VMS exit status, instead of the default emulation of POSIX
579 Also see L<Error Indicators>.
587 If used numerically, yields the current value of the C C<errno>
588 variable, with all the usual caveats. (This means that you shouldn't
589 depend on the value of C<$!> to be anything in particular unless
590 you've gotten a specific error return indicating a system error.)
591 If used an a string, yields the corresponding system error string.
592 You can assign a number to C<$!> to set I<errno> if, for instance,
593 you want C<"$!"> to return the string for error I<n>, or you want
594 to set the exit value for the die() operator. (Mnemonic: What just
597 Also see L<Error Indicators>.
599 =item $EXTENDED_OS_ERROR
603 Error information specific to the current operating system. At
604 the moment, this differs from C<$!> under only VMS, OS/2, and Win32
605 (and for MacPerl). On all other platforms, C<$^E> is always just
608 Under VMS, C<$^E> provides the VMS status value from the last
609 system error. This is more specific information about the last
610 system error than that provided by C<$!>. This is particularly
611 important when C<$!> is set to B<EVMSERR>.
613 Under OS/2, C<$^E> is set to the error code of the last call to
614 OS/2 API either via CRT, or directly from perl.
616 Under Win32, C<$^E> always returns the last error information
617 reported by the Win32 call C<GetLastError()> which describes
618 the last error from within the Win32 API. Most Win32-specific
619 code will report errors via C<$^E>. ANSI C and Unix-like calls
620 set C<errno> and so most portable Perl code will report errors
623 Caveats mentioned in the description of C<$!> generally apply to
624 C<$^E>, also. (Mnemonic: Extra error explanation.)
626 Also see L<Error Indicators>.
632 The Perl syntax error message from the last eval() operator.
633 If $@ is the null string, the last eval() parsed and executed
634 correctly (although the operations you invoked may have failed in the
635 normal fashion). (Mnemonic: Where was the syntax error "at"?)
637 Warning messages are not collected in this variable. You can,
638 however, set up a routine to process warnings by setting C<$SIG{__WARN__}>
641 Also see L<Error Indicators>.
649 The process number of the Perl running this script. You should
650 consider this variable read-only, although it will be altered
651 across fork() calls. (Mnemonic: same as shells.)
659 The real uid of this process. (Mnemonic: it's the uid you came I<from>,
660 if you're running setuid.) You can change both the real uid and
661 the effective uid at the same time by using POSIX::setuid().
663 =item $EFFECTIVE_USER_ID
669 The effective uid of this process. Example:
671 $< = $>; # set real to effective uid
672 ($<,$>) = ($>,$<); # swap real and effective uid
674 You can change both the effective uid and the real uid at the same
675 time by using POSIX::setuid().
677 (Mnemonic: it's the uid you went I<to>, if you're running setuid.)
678 C<< $< >> and C<< $> >> can be swapped only on machines
679 supporting setreuid().
687 The real gid of this process. If you are on a machine that supports
688 membership in multiple groups simultaneously, gives a space separated
689 list of groups you are in. The first number is the one returned by
690 getgid(), and the subsequent ones by getgroups(), one of which may be
691 the same as the first number.
693 However, a value assigned to C<$(> must be a single number used to
694 set the real gid. So the value given by C<$(> should I<not> be assigned
695 back to C<$(> without being forced numeric, such as by adding zero.
697 You can change both the real gid and the effective gid at the same
698 time by using POSIX::setgid().
700 (Mnemonic: parentheses are used to I<group> things. The real gid is the
701 group you I<left>, if you're running setgid.)
703 =item $EFFECTIVE_GROUP_ID
709 The effective gid of this process. If you are on a machine that
710 supports membership in multiple groups simultaneously, gives a space
711 separated list of groups you are in. The first number is the one
712 returned by getegid(), and the subsequent ones by getgroups(), one of
713 which may be the same as the first number.
715 Similarly, a value assigned to C<$)> must also be a space-separated
716 list of numbers. The first number sets the effective gid, and
717 the rest (if any) are passed to setgroups(). To get the effect of an
718 empty list for setgroups(), just repeat the new effective gid; that is,
719 to force an effective gid of 5 and an effectively empty setgroups()
720 list, say C< $) = "5 5" >.
722 You can change both the effective gid and the real gid at the same
723 time by using POSIX::setgid() (use only a single numeric argument).
725 (Mnemonic: parentheses are used to I<group> things. The effective gid
726 is the group that's I<right> for you, if you're running setgid.)
728 C<< $< >>, C<< $> >>, C<$(> and C<$)> can be set only on
729 machines that support the corresponding I<set[re][ug]id()> routine. C<$(>
730 and C<$)> can be swapped only on machines supporting setregid().
736 Contains the name of the program being executed. On some operating
737 systems assigning to C<$0> modifies the argument area that the B<ps>
738 program sees. This is more useful as a way of indicating the current
739 program state than it is for hiding the program you're running.
740 (Mnemonic: same as B<sh> and B<ksh>.)
742 Note for BSD users: setting C<$0> does not completely remove "perl"
743 from the ps(1) output. For example, setting C<$0> to C<"foobar"> will
744 result in C<"perl: foobar (perl)">. This is an operating system
749 The index of the first element in an array, and of the first character
750 in a substring. Default is 0, but you could theoretically set it
751 to 1 to make Perl behave more like B<awk> (or Fortran) when
752 subscripting and when evaluating the index() and substr() functions.
753 (Mnemonic: [ begins subscripts.)
755 As of release 5 of Perl, assignment to C<$[> is treated as a compiler
756 directive, and cannot influence the behavior of any other file.
757 Its use is highly discouraged.
761 The version + patchlevel / 1000 of the Perl interpreter. This variable
762 can be used to determine whether the Perl interpreter executing a
763 script is in the right range of versions. (Mnemonic: Is this version
764 of perl in the right bracket?) Example:
766 warn "No checksumming!\n" if $] < 3.019;
768 See also the documentation of C<use VERSION> and C<require VERSION>
769 for a convenient way to fail if the running Perl interpreter is too old.
771 The use of this variable is deprecated. The floating point representation
772 can sometimes lead to inaccurate numeric comparisons. See C<$^V> for a
773 more modern representation of the Perl version that allows accurate string
780 The current value of the flag associated with the B<-c> switch.
781 Mainly of use with B<-MO=...> to allow code to alter its behavior
782 when being compiled, such as for example to AUTOLOAD at compile
783 time rather than normal, deferred loading. See L<perlcc>. Setting
784 C<$^C = 1> is similar to calling C<B::minus_c>.
790 The current value of the debugging flags. (Mnemonic: value of B<-D>
797 The maximum system file descriptor, ordinarily 2. System file
798 descriptors are passed to exec()ed processes, while higher file
799 descriptors are not. Also, during an open(), system file descriptors are
800 preserved even if the open() fails. (Ordinary file descriptors are
801 closed before the open() is attempted.) The close-on-exec
802 status of a file descriptor will be decided according to the value of
803 C<$^F> when the corresponding file, pipe, or socket was opened, not the
808 WARNING: This variable is strictly for internal use only. Its availability,
809 behavior, and contents are subject to change without notice.
811 This variable contains compile-time hints for the Perl interpreter. At the
812 end of compilation of a BLOCK the value of this variable is restored to the
813 value when the interpreter started to compile the BLOCK.
815 When perl begins to parse any block construct that provides a lexical scope
816 (e.g., eval body, required file, subroutine body, loop body, or conditional
817 block), the existing value of $^H is saved, but its value is left unchanged.
818 When the compilation of the block is completed, it regains the saved value.
819 Between the points where its value is saved and restored, code that
820 executes within BEGIN blocks is free to change the value of $^H.
822 This behavior provides the semantic of lexical scoping, and is used in,
823 for instance, the C<use strict> pragma.
825 The contents should be an integer; different bits of it are used for
826 different pragmatic flags. Here's an example:
828 sub add_100 { $^H |= 0x100 }
835 Consider what happens during execution of the BEGIN block. At this point
836 the BEGIN block has already been compiled, but the body of foo() is still
837 being compiled. The new value of $^H will therefore be visible only while
838 the body of foo() is being compiled.
840 Substitution of the above BEGIN block with:
842 BEGIN { require strict; strict->import('vars') }
844 demonstrates how C<use strict 'vars'> is implemented. Here's a conditional
845 version of the same lexical pragma:
847 BEGIN { require strict; strict->import('vars') if $condition }
851 WARNING: This variable is strictly for internal use only. Its availability,
852 behavior, and contents are subject to change without notice.
854 The %^H hash provides the same scoping semantic as $^H. This makes it
855 useful for implementation of lexically scoped pragmas.
861 The current value of the inplace-edit extension. Use C<undef> to disable
862 inplace editing. (Mnemonic: value of B<-i> switch.)
866 By default, running out of memory is an untrappable, fatal error.
867 However, if suitably built, Perl can use the contents of C<$^M>
868 as an emergency memory pool after die()ing. Suppose that your Perl
869 were compiled with -DPERL_EMERGENCY_SBRK and used Perl's malloc.
872 $^M = 'a' x (1 << 16);
874 would allocate a 64K buffer for use in an emergency. See the
875 F<INSTALL> file in the Perl distribution for information on how to
876 enable this option. To discourage casual use of this advanced
877 feature, there is no L<English|English> long name for this variable.
883 The name of the operating system under which this copy of Perl was
884 built, as determined during the configuration process. The value
885 is identical to C<$Config{'osname'}>. See also L<Config> and the
886 B<-V> command-line switch documented in L<perlrun>.
892 The internal variable for debugging support. The meanings of the
893 various bits are subject to change, but currently indicate:
899 Debug subroutine enter/exit.
903 Line-by-line debugging.
907 Switch off optimizations.
911 Preserve more data for future interactive inspections.
915 Keep info about source lines on which a subroutine is defined.
919 Start with single-step on.
923 Use subroutine address instead of name when reporting.
927 Report C<goto &subroutine> as well.
931 Provide informative "file" names for evals based on the place they were compiled.
935 Provide informative names to anonymous subroutines based on the place they
940 Some bits may be relevant at compile-time only, some at
941 run-time only. This is a new mechanism and the details may change.
943 =item $LAST_REGEXP_CODE_RESULT
947 The result of evaluation of the last successful C<(?{ code })>
948 regular expression assertion (see L<perlre>). May be written to.
950 =item $EXCEPTIONS_BEING_CAUGHT
954 Current state of the interpreter. Undefined if parsing of the current
955 module/eval is not finished (may happen in $SIG{__DIE__} and
956 $SIG{__WARN__} handlers). True if inside an eval(), otherwise false.
962 The time at which the program began running, in seconds since the
963 epoch (beginning of 1970). The values returned by the B<-M>, B<-A>,
964 and B<-C> filetests are based on this value.
970 The revision, version, and subversion of the Perl interpreter, represented
971 as a string composed of characters with those ordinals. Thus in Perl v5.6.0
972 it equals C<chr(5) . chr(6) . chr(0)> and will return true for
973 C<$^V eq v5.6.0>. Note that the characters in this string value can
974 potentially be in Unicode range.
976 This can be used to determine whether the Perl interpreter executing a
977 script is in the right range of versions. (Mnemonic: use ^V for Version
980 warn "No \"our\" declarations!\n" if $^V and $^V lt v5.6.0;
982 See the documentation of C<use VERSION> and C<require VERSION>
983 for a convenient way to fail if the running Perl interpreter is too old.
985 See also C<$]> for an older representation of the Perl version.
991 The current value of the warning switch, initially true if B<-w>
992 was used, false otherwise, but directly modifiable. (Mnemonic:
993 related to the B<-w> switch.) See also L<warnings>.
995 =item ${^WARNING_BITS}
997 The current set of warning checks enabled by the C<use warnings> pragma.
998 See the documentation of C<warnings> for more details.
1000 =item ${^WIDE_SYSTEM_CALLS}
1002 Global flag that enables system calls made by Perl to use wide character
1003 APIs native to the system, if available. This is currently only implemented
1004 on the Windows platform.
1006 This can also be enabled from the command line using the C<-C> switch.
1008 The initial value is typically C<0> for compatibility with Perl versions
1009 earlier than 5.6, but may be automatically set to C<1> by Perl if the system
1010 provides a user-settable default (e.g., C<$ENV{LC_CTYPE}>).
1012 The C<bytes> pragma always overrides the effect of this flag in the current
1013 lexical scope. See L<bytes>.
1015 =item $EXECUTABLE_NAME
1019 The name that the Perl binary itself was executed as, from C's C<argv[0]>.
1020 This may not be a full pathname, nor even necessarily in your path.
1024 contains the name of the current file when reading from <>.
1028 The array @ARGV contains the command-line arguments intended for
1029 the script. C<$#ARGV> is generally the number of arguments minus
1030 one, because C<$ARGV[0]> is the first argument, I<not> the program's
1031 command name itself. See C<$0> for the command name.
1035 The array @F contains the fields of each line read in when autosplit
1036 mode is turned on. See L<perlrun> for the B<-a> switch. This array
1037 is package-specific, and must be declared or given a full package name
1038 if not in package main when running under C<strict 'vars'>.
1042 The array @INC contains the list of places that the C<do EXPR>,
1043 C<require>, or C<use> constructs look for their library files. It
1044 initially consists of the arguments to any B<-I> command-line
1045 switches, followed by the default Perl library, probably
1046 F</usr/local/lib/perl>, followed by ".", to represent the current
1047 directory. If you need to modify this at runtime, you should use
1048 the C<use lib> pragma to get the machine-dependent library properly
1051 use lib '/mypath/libdir/';
1056 Within a subroutine the array @_ contains the parameters passed to that
1057 subroutine. See L<perlsub>.
1061 The hash %INC contains entries for each filename included via the
1062 C<do>, C<require>, or C<use> operators. The key is the filename
1063 you specified (with module names converted to pathnames), and the
1064 value is the location of the file found. The C<require>
1065 operator uses this hash to determine whether a particular file has
1066 already been included.
1072 The hash %ENV contains your current environment. Setting a
1073 value in C<ENV> changes the environment for any child processes
1074 you subsequently fork() off.
1080 The hash %SIG contains signal handlers for signals. For example:
1082 sub handler { # 1st argument is signal name
1084 print "Caught a SIG$sig--shutting down\n";
1089 $SIG{'INT'} = \&handler;
1090 $SIG{'QUIT'} = \&handler;
1092 $SIG{'INT'} = 'DEFAULT'; # restore default action
1093 $SIG{'QUIT'} = 'IGNORE'; # ignore SIGQUIT
1095 Using a value of C<'IGNORE'> usually has the effect of ignoring the
1096 signal, except for the C<CHLD> signal. See L<perlipc> for more about
1099 Here are some other examples:
1101 $SIG{"PIPE"} = "Plumber"; # assumes main::Plumber (not recommended)
1102 $SIG{"PIPE"} = \&Plumber; # just fine; assume current Plumber
1103 $SIG{"PIPE"} = *Plumber; # somewhat esoteric
1104 $SIG{"PIPE"} = Plumber(); # oops, what did Plumber() return??
1106 Be sure not to use a bareword as the name of a signal handler,
1107 lest you inadvertently call it.
1109 If your system has the sigaction() function then signal handlers are
1110 installed using it. This means you get reliable signal handling. If
1111 your system has the SA_RESTART flag it is used when signals handlers are
1112 installed. This means that system calls for which restarting is supported
1113 continue rather than returning when a signal arrives. If you want your
1114 system calls to be interrupted by signal delivery then do something like
1117 use POSIX ':signal_h';
1120 sigaction SIGALRM, new POSIX::SigAction sub { $alarm = 1 }
1121 or die "Error setting SIGALRM handler: $!\n";
1125 Certain internal hooks can be also set using the %SIG hash. The
1126 routine indicated by C<$SIG{__WARN__}> is called when a warning message is
1127 about to be printed. The warning message is passed as the first
1128 argument. The presence of a __WARN__ hook causes the ordinary printing
1129 of warnings to STDERR to be suppressed. You can use this to save warnings
1130 in a variable, or turn warnings into fatal errors, like this:
1132 local $SIG{__WARN__} = sub { die $_[0] };
1135 The routine indicated by C<$SIG{__DIE__}> is called when a fatal exception
1136 is about to be thrown. The error message is passed as the first
1137 argument. When a __DIE__ hook routine returns, the exception
1138 processing continues as it would have in the absence of the hook,
1139 unless the hook routine itself exits via a C<goto>, a loop exit, or a die().
1140 The C<__DIE__> handler is explicitly disabled during the call, so that you
1141 can die from a C<__DIE__> handler. Similarly for C<__WARN__>.
1143 Due to an implementation glitch, the C<$SIG{__DIE__}> hook is called
1144 even inside an eval(). Do not use this to rewrite a pending exception
1145 in C<$@>, or as a bizarre substitute for overriding CORE::GLOBAL::die().
1146 This strange action at a distance may be fixed in a future release
1147 so that C<$SIG{__DIE__}> is only called if your program is about
1148 to exit, as was the original intent. Any other use is deprecated.
1150 C<__DIE__>/C<__WARN__> handlers are very special in one respect:
1151 they may be called to report (probable) errors found by the parser.
1152 In such a case the parser may be in inconsistent state, so any
1153 attempt to evaluate Perl code from such a handler will probably
1154 result in a segfault. This means that warnings or errors that
1155 result from parsing Perl should be used with extreme caution, like
1158 require Carp if defined $^S;
1159 Carp::confess("Something wrong") if defined &Carp::confess;
1160 die "Something wrong, but could not load Carp to give backtrace...
1161 To see backtrace try starting Perl with -MCarp switch";
1163 Here the first line will load Carp I<unless> it is the parser who
1164 called the handler. The second line will print backtrace and die if
1165 Carp was available. The third line will be executed only if Carp was
1168 See L<perlfunc/die>, L<perlfunc/warn>, L<perlfunc/eval>, and
1169 L<warnings> for additional information.
1173 =head2 Error Indicators
1175 The variables C<$@>, C<$!>, C<$^E>, and C<$?> contain information
1176 about different types of error conditions that may appear during
1177 execution of a Perl program. The variables are shown ordered by
1178 the "distance" between the subsystem which reported the error and
1179 the Perl process. They correspond to errors detected by the Perl
1180 interpreter, C library, operating system, or an external program,
1183 To illustrate the differences between these variables, consider the
1184 following Perl expression, which uses a single-quoted string:
1187 open PIPE, "/cdrom/install |";
1189 close PIPE or die "bad pipe: $?, $!";
1192 After execution of this statement all 4 variables may have been set.
1194 C<$@> is set if the string to be C<eval>-ed did not compile (this
1195 may happen if C<open> or C<close> were imported with bad prototypes),
1196 or if Perl code executed during evaluation die()d . In these cases
1197 the value of $@ is the compile error, or the argument to C<die>
1198 (which will interpolate C<$!> and C<$?>!). (See also L<Fatal>,
1201 When the eval() expression above is executed, open(), C<< <PIPE> >>,
1202 and C<close> are translated to calls in the C run-time library and
1203 thence to the operating system kernel. C<$!> is set to the C library's
1204 C<errno> if one of these calls fails.
1206 Under a few operating systems, C<$^E> may contain a more verbose
1207 error indicator, such as in this case, "CDROM tray not closed."
1208 Systems that do not support extended error messages leave C<$^E>
1211 Finally, C<$?> may be set to non-0 value if the external program
1212 F</cdrom/install> fails. The upper eight bits reflect specific
1213 error conditions encountered by the program (the program's exit()
1214 value). The lower eight bits reflect mode of failure, like signal
1215 death and core dump information See wait(2) for details. In
1216 contrast to C<$!> and C<$^E>, which are set only if error condition
1217 is detected, the variable C<$?> is set on each C<wait> or pipe
1218 C<close>, overwriting the old value. This is more like C<$@>, which
1219 on every eval() is always set on failure and cleared on success.
1221 For more details, see the individual descriptions at C<$@>, C<$!>, C<$^E>,
1224 =head2 Technical Note on the Syntax of Variable Names
1226 Variable names in Perl can have several formats. Usually, they
1227 must begin with a letter or underscore, in which case they can be
1228 arbitrarily long (up to an internal limit of 251 characters) and
1229 may contain letters, digits, underscores, or the special sequence
1230 C<::> or C<'>. In this case, the part before the last C<::> or
1231 C<'> is taken to be a I<package qualifier>; see L<perlmod>.
1233 Perl variable names may also be a sequence of digits or a single
1234 punctuation or control character. These names are all reserved for
1235 special uses by Perl; for example, the all-digits names are used
1236 to hold data captured by backreferences after a regular expression
1237 match. Perl has a special syntax for the single-control-character
1238 names: It understands C<^X> (caret C<X>) to mean the control-C<X>
1239 character. For example, the notation C<$^W> (dollar-sign caret
1240 C<W>) is the scalar variable whose name is the single character
1241 control-C<W>. This is better than typing a literal control-C<W>
1244 Finally, new in Perl 5.6, Perl variable names may be alphanumeric
1245 strings that begin with control characters (or better yet, a caret).
1246 These variables must be written in the form C<${^Foo}>; the braces
1247 are not optional. C<${^Foo}> denotes the scalar variable whose
1248 name is a control-C<F> followed by two C<o>'s. These variables are
1249 reserved for future special uses by Perl, except for the ones that
1250 begin with C<^_> (control-underscore or caret-underscore). No
1251 control-character name that begins with C<^_> will acquire a special
1252 meaning in any future version of Perl; such names may therefore be
1253 used safely in programs. C<$^_> itself, however, I<is> reserved.
1255 Perl identifiers that begin with digits, control characters, or
1256 punctuation characters are exempt from the effects of the C<package>
1257 declaration and are always forced to be in package C<main>. A few
1258 other names are also exempt:
1266 In particular, the new special C<${^_XYZ}> variables are always taken
1267 to be in package C<main>, regardless of any C<package> declarations
1272 Due to an unfortunate accident of Perl's implementation, C<use
1273 English> imposes a considerable performance penalty on all regular
1274 expression matches in a program, regardless of whether they occur
1275 in the scope of C<use English>. For that reason, saying C<use
1276 English> in libraries is strongly discouraged. See the
1277 Devel::SawAmpersand module documentation from CPAN
1278 (http://www.perl.com/CPAN/modules/by-module/Devel/)
1279 for more information.
1281 Having to even think about the C<$^S> variable in your exception
1282 handlers is simply wrong. C<$SIG{__DIE__}> as currently implemented
1283 invites grievous and difficult to track down errors. Avoid it
1284 and use an C<END{}> or CORE::GLOBAL::die override instead.