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1=head1 NAME
2
3perlvar - Perl predefined variables
4
5=head1 DESCRIPTION
6
7=head2 Predefined Names
8
5a964f20 9The following names have special meaning to Perl. Most
5f05dabc 10punctuation names have reasonable mnemonics, or analogues in one of
5a964f20 11the shells. Nevertheless, if you wish to use long variable names,
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12you just need to say
13
14 use English;
15
16at the top of your program. This will alias all the short names to the
5a964f20 17long names in the current package. Some even have medium names,
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18generally borrowed from B<awk>.
19
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20If you don't mind the performance hit, variables that depend on the
21currently selected filehandle may instead be set by calling an
22appropriate object method on the IO::Handle object. (Summary lines
23below for this contain the word HANDLE.) First you must say
a0d0e21e 24
19799a22 25 use IO::Handle;
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26
27after which you may use either
28
29 method HANDLE EXPR
30
5a964f20 31or more safely,
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32
33 HANDLE->method(EXPR)
34
19799a22 35Each of the methods returns the old value of the IO::Handle attribute.
a0d0e21e 36The methods each take an optional EXPR, which if supplied specifies the
19799a22 37new value for the IO::Handle attribute in question. If not supplied,
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38most of the methods do nothing to the current value, except for
39autoflush(), which will assume a 1 for you, just to be different.
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40It costs quite a bit to load in the IO::Handle class, so you should
41learn how to use the regular built-in variables.
a0d0e21e 42
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43A few of these variables are considered "read-only". This means that if
44you try to assign to this variable, either directly or indirectly through
45a reference, you'll raise a run-time exception.
a0d0e21e 46
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47The following list is ordered by scalar variables first, then the
48arrays, then the hashes (except $^M was added in the wrong place).
49This is somewhat obscured by the fact that %ENV and %SIG are listed as
50$ENV{expr} and $SIG{expr}.
51
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52=over 8
53
54=item $ARG
55
56=item $_
57
58The default input and pattern-searching space. The following pairs are
59equivalent:
60
19799a22 61 while (<>) {...} # equivalent only in while!
54310121 62 while (defined($_ = <>)) {...}
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63
64 /^Subject:/
65 $_ =~ /^Subject:/
66
67 tr/a-z/A-Z/
68 $_ =~ tr/a-z/A-Z/
69
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70 chomp
71 chomp($_)
a0d0e21e 72
54310121 73Here are the places where Perl will assume $_ even if you
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74don't use it:
75
76=over 3
77
78=item *
79
80Various unary functions, including functions like ord() and int(), as well
81as the all file tests (C<-f>, C<-d>) except for C<-t>, which defaults to
82STDIN.
83
84=item *
85
86Various list functions like print() and unlink().
87
88=item *
89
90The pattern matching operations C<m//>, C<s///>, and C<tr///> when used
91without an C<=~> operator.
92
54310121 93=item *
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94
95The default iterator variable in a C<foreach> loop if no other
96variable is supplied.
97
54310121 98=item *
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99
100The implicit iterator variable in the grep() and map() functions.
101
54310121 102=item *
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103
104The default place to put an input record when a C<E<lt>FHE<gt>>
105operation's result is tested by itself as the sole criterion of a C<while>
19799a22 106test. Outside of a C<while> test, this will not happen.
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107
108=back
109
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110(Mnemonic: underline is understood in certain operations.)
111
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112=back
113
114=over 8
115
5a964f20 116=item $E<lt>I<digits>E<gt>
a0d0e21e 117
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118Contains the subpattern from the corresponding set of capturing
119parentheses from the last pattern match, not counting patterns
120matched in nested blocks that have been exited already. (Mnemonic:
121like \digits.) These variables are all read-only and dynamically
122scoped to the current BLOCK.
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123
124=item $MATCH
125
126=item $&
127
128The string matched by the last successful pattern match (not counting
129any matches hidden within a BLOCK or eval() enclosed by the current
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130BLOCK). (Mnemonic: like & in some editors.) This variable is read-only
131and dynamically scoped to the current BLOCK.
a0d0e21e 132
19ddd453 133The use of this variable anywhere in a program imposes a considerable
19799a22 134performance penalty on all regular expression matches. See L<BUGS>.
19ddd453 135
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136=item $PREMATCH
137
138=item $`
139
140The string preceding whatever was matched by the last successful
141pattern match (not counting any matches hidden within a BLOCK or eval
a8f8344d 142enclosed by the current BLOCK). (Mnemonic: C<`> often precedes a quoted
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143string.) This variable is read-only.
144
19ddd453 145The use of this variable anywhere in a program imposes a considerable
19799a22 146performance penalty on all regular expression matches. See L<BUGS>.
19ddd453 147
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148=item $POSTMATCH
149
150=item $'
151
152The string following whatever was matched by the last successful
153pattern match (not counting any matches hidden within a BLOCK or eval()
a8f8344d 154enclosed by the current BLOCK). (Mnemonic: C<'> often follows a quoted
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155string.) Example:
156
157 $_ = 'abcdefghi';
158 /def/;
159 print "$`:$&:$'\n"; # prints abc:def:ghi
160
19799a22 161This variable is read-only and dynamically scoped to the current BLOCK.
a0d0e21e 162
19ddd453 163The use of this variable anywhere in a program imposes a considerable
19799a22 164performance penalty on all regular expression matches. See L<BUGS>.
19ddd453 165
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166=item $LAST_PAREN_MATCH
167
168=item $+
169
170The last bracket matched by the last search pattern. This is useful if
19799a22 171you don't know which one of a set of alternative patterns matched. For
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172example:
173
174 /Version: (.*)|Revision: (.*)/ && ($rev = $+);
175
176(Mnemonic: be positive and forward looking.)
19799a22 177This variable is read-only and dynamically scoped to the current BLOCK.
a0d0e21e 178
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179=item @+
180
19799a22 181$+[0] is the offset of the end of the last successful match.
6cef1e77 182C<$+[>I<n>C<]> is the offset of the end of the substring matched by
8f580fb8 183I<n>-th subpattern, or undef if the subpattern did not match.
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184
185Thus after a match against $_, $& coincides with C<substr $_, $-[0],
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186$+[0] - $-[0]>. Similarly, C<$>I<n> coincides with C<substr $_, $-[>I<n>C<],
187$+[>I<n>C<] - $-[>I<n>C<]> if C<$-[>I<n>C<]> is defined, and $+ coincides with
188C<substr $_, $-[$#-], $+[$#-]>. One can use C<$#+> to find the number
189of subgroups in the last successful match. Note the difference with
19799a22 190C<$#->, which is the last I<matched> subgroup. Compare with C<@->.
6cef1e77 191
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192=item $MULTILINE_MATCHING
193
194=item $*
195
4a6725af 196Set to 1 to do multi-line matching within a string, 0 to tell Perl
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197that it can assume that strings contain a single line, for the purpose
198of optimizing pattern matches. Pattern matches on strings containing
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199multiple newlines can produce confusing results when C<$*> is 0. Default
200is 0. (Mnemonic: * matches multiple things.) This variable
201influences the interpretation of only C<^> and C<$>. A literal newline can
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202be searched for even when C<$* == 0>.
203
19799a22 204Use of C<$*> is deprecated in modern Perl, supplanted by
5a964f20 205the C</s> and C</m> modifiers on pattern matching.
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206
207=item input_line_number HANDLE EXPR
208
209=item $INPUT_LINE_NUMBER
210
211=item $NR
212
213=item $.
214
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215The current input record number for the last file handle from which
216you just read() (or performed a C<seek> or C<tell> on). The value
883faa13 217may be different from the actual physical line number in the file,
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218depending on what notion of "line" is in effect--see C<$/> on how
219to change that. An explicit close on a filehandle resets the line
220number. Because C<E<lt>E<gt>> never does an explicit close, line
221numbers increase across ARGV files (but see examples in L<perlfunc/eof>).
222Consider this variable read-only: setting it does not reposition
223the seek pointer; you'll have to do that on your own. (Mnemonic:
224many programs use "." to mean the current line number.)
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225
226=item input_record_separator HANDLE EXPR
227
228=item $INPUT_RECORD_SEPARATOR
229
230=item $RS
231
232=item $/
233
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234The input record separator, newline by default. This is used to
235influence Perl's idea of what a "line" is. Works like B<awk>'s RS
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236variable, including treating empty lines as a terminator if set to
237the null string. (Note: An empty line cannot contain any spaces
238or tabs.) You may set it to a multi-character string to match a
239multi-character terminator, or to C<undef> to read through the end
240of file. Setting it to C<"\n\n"> means something slightly
241different than setting to C<"">, if the file contains consecutive
242empty lines. Setting to C<""> will treat two or more consecutive
243empty lines as a single empty line. Setting to C<"\n\n"> will
244blindly assume that the next input character belongs to the next
245paragraph, even if it's a newline. (Mnemonic: / is used to delimit
246line boundaries when quoting poetry.)
a0d0e21e 247
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248 undef $/; # enable "slurp" mode
249 $_ = <FH>; # whole file now here
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250 s/\n[ \t]+/ /g;
251
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252Remember: the value of C<$/> is a string, not a regex. B<awk> has to be
253better for something. :-)
68dc0745 254
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255Setting C<$/> to a reference to an integer, scalar containing an integer, or
256scalar that's convertible to an integer will attempt to read records
5b2b9c68 257instead of lines, with the maximum record size being the referenced
19799a22 258integer. So this:
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259
260 $/ = \32768; # or \"32768", or \$var_containing_32768
261 open(FILE, $myfile);
262 $_ = <FILE>;
263
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264will read a record of no more than 32768 bytes from FILE. If you're
265not reading from a record-oriented file (or your OS doesn't have
266record-oriented files), then you'll likely get a full chunk of data
267with every read. If a record is larger than the record size you've
268set, you'll get the record back in pieces.
5b2b9c68 269
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270On VMS, record reads are done with the equivalent of C<sysread>,
271so it's best not to mix record and non-record reads on the same
272file. (This is unlikely to be a problem, because any file you'd
273want to read in record mode is probably usable in line mode.)
274Non-VMS systems perform normal I/O, so it's safe to mix record and
275non-record reads of a file.
5b2b9c68 276
19799a22 277Also see C<$.>.
883faa13 278
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279=item autoflush HANDLE EXPR
280
281=item $OUTPUT_AUTOFLUSH
282
283=item $|
284
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285If set to nonzero, forces a flush right away and after every write
286or print on the currently selected output channel. Default is 0
287(regardless of whether the channel is actually buffered by the
288system or not; C<$|> tells you only whether you've asked Perl
289explicitly to flush after each write). STDOUT will
290typically be line buffered if output is to the terminal and block
291buffered otherwise. Setting this variable is useful primarily when
292you are outputting to a pipe or socket, such as when you are running
293a Perl program under B<rsh> and want to see the output as it's
294happening. This has no effect on input buffering. See L<perlfunc/getc>
295for that. (Mnemonic: when you want your pipes to be piping hot.)
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296
297=item output_field_separator HANDLE EXPR
298
299=item $OUTPUT_FIELD_SEPARATOR
300
301=item $OFS
302
303=item $,
304
305The output field separator for the print operator. Ordinarily the
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306print operator simply prints out its arguments without further
307adornment. To get behavior more like B<awk>, set this variable as
308you would set B<awk>'s OFS variable to specify what is printed
309between fields. (Mnemonic: what is printed when there is a "," in
310your print statement.)
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311
312=item output_record_separator HANDLE EXPR
313
314=item $OUTPUT_RECORD_SEPARATOR
315
316=item $ORS
317
318=item $\
319
320The output record separator for the print operator. Ordinarily the
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321print operator simply prints out its arguments as is, with no
322trailing newline or other end-of-record string added. To get
323behavior more like B<awk>, set this variable as you would set
324B<awk>'s ORS variable to specify what is printed at the end of the
325print. (Mnemonic: you set C<$\> instead of adding "\n" at the
326end of the print. Also, it's just like C<$/>, but it's what you
327get "back" from Perl.)
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328
329=item $LIST_SEPARATOR
330
331=item $"
332
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333This is like C<$,> except that it applies to array and slice values
334interpolated into a double-quoted string (or similar interpreted
335string). Default is a space. (Mnemonic: obvious, I think.)
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336
337=item $SUBSCRIPT_SEPARATOR
338
339=item $SUBSEP
340
341=item $;
342
54310121 343The subscript separator for multidimensional array emulation. If you
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344refer to a hash element as
345
346 $foo{$a,$b,$c}
347
348it really means
349
350 $foo{join($;, $a, $b, $c)}
351
352But don't put
353
354 @foo{$a,$b,$c} # a slice--note the @
355
356which means
357
358 ($foo{$a},$foo{$b},$foo{$c})
359
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360Default is "\034", the same as SUBSEP in B<awk>. If your
361keys contain binary data there might not be any safe value for C<$;>.
a0d0e21e 362(Mnemonic: comma (the syntactic subscript separator) is a
19799a22 363semi-semicolon. Yeah, I know, it's pretty lame, but C<$,> is already
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364taken for something more important.)
365
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366Consider using "real" multidimensional arrays as described
367in L<perllol>.
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368
369=item $OFMT
370
371=item $#
372
373The output format for printed numbers. This variable is a half-hearted
374attempt to emulate B<awk>'s OFMT variable. There are times, however,
375when B<awk> and Perl have differing notions of what is in fact
19799a22 376numeric. The initial value is "%.I<n>g", where I<n> is the value
6e2995f4 377of the macro DBL_DIG from your system's F<float.h>. This is different from
19799a22 378B<awk>'s default OFMT setting of "%.6g", so you need to set C<$#>
6e2995f4 379explicitly to get B<awk>'s value. (Mnemonic: # is the number sign.)
a0d0e21e 380
19799a22 381Use of C<$#> is deprecated.
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382
383=item format_page_number HANDLE EXPR
384
385=item $FORMAT_PAGE_NUMBER
386
387=item $%
388
389The current page number of the currently selected output channel.
19799a22 390Used with formats.
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391(Mnemonic: % is page number in B<nroff>.)
392
393=item format_lines_per_page HANDLE EXPR
394
395=item $FORMAT_LINES_PER_PAGE
396
397=item $=
398
399The current page length (printable lines) of the currently selected
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400output channel. Default is 60.
401Used with formats.
402(Mnemonic: = has horizontal lines.)
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403
404=item format_lines_left HANDLE EXPR
405
406=item $FORMAT_LINES_LEFT
407
408=item $-
409
410The number of lines left on the page of the currently selected output
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411channel.
412Used with formats.
413(Mnemonic: lines_on_page - lines_printed.)
a0d0e21e 414
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415=item @-
416
19799a22 417$-[0] is the offset of the start of the last successful match.
6cef1e77 418C<$-[>I<n>C<]> is the offset of the start of the substring matched by
8f580fb8 419I<n>-th subpattern, or undef if the subpattern did not match.
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420
421Thus after a match against $_, $& coincides with C<substr $_, $-[0],
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422$+[0] - $-[0]>. Similarly, C<$>I<n> coincides with C<substr $_, $-[>I<n>C<],
423$+[>I<n>C<] - $-[>I<n>C<]> if C<$-[>I<n>C<]> is defined, and $+ coincides with
424C<substr $_, $-[$#-], $+[$#-]>. One can use C<$#-> to find the last
425matched subgroup in the last successful match. Note the difference with
426C<$#+>, which is the number of subgroups in the regular expression. Compare
19799a22 427with C<@+>.
6cef1e77 428
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429=item format_name HANDLE EXPR
430
431=item $FORMAT_NAME
432
433=item $~
434
435The name of the current report format for the currently selected output
436channel. Default is name of the filehandle. (Mnemonic: brother to
19799a22 437C<$^>.)
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438
439=item format_top_name HANDLE EXPR
440
441=item $FORMAT_TOP_NAME
442
443=item $^
444
445The name of the current top-of-page format for the currently selected
446output channel. Default is name of the filehandle with _TOP
447appended. (Mnemonic: points to top of page.)
448
449=item format_line_break_characters HANDLE EXPR
450
451=item $FORMAT_LINE_BREAK_CHARACTERS
452
453=item $:
454
455The current set of characters after which a string may be broken to
54310121 456fill continuation fields (starting with ^) in a format. Default is
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457S<" \n-">, to break on whitespace or hyphens. (Mnemonic: a "colon" in
458poetry is a part of a line.)
459
460=item format_formfeed HANDLE EXPR
461
462=item $FORMAT_FORMFEED
463
464=item $^L
465
5f05dabc 466What formats output to perform a form feed. Default is \f.
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467
468=item $ACCUMULATOR
469
470=item $^A
471
472The current value of the write() accumulator for format() lines. A format
19799a22 473contains formline() calls that put their result into C<$^A>. After
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474calling its format, write() prints out the contents of C<$^A> and empties.
475So you never actually see the contents of C<$^A> unless you call
476formline() yourself and then look at it. See L<perlform> and
477L<perlfunc/formline()>.
478
479=item $CHILD_ERROR
480
481=item $?
482
54310121 483The status returned by the last pipe close, backtick (C<``>) command,
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484successful call to wait() or waitpid(), or from the system()
485operator. This is just the 16-bit status word returned by the
486wait() system call (or else is made up to look like it). Thus, the
487exit value of the subprocess is actually (C<$? E<gt>E<gt> 8>), and
488C<$? & 127> gives which signal, if any, the process died from, and
489C<$? & 128> reports whether there was a core dump. (Mnemonic:
490similar to B<sh> and B<ksh>.)
a0d0e21e 491
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492Additionally, if the C<h_errno> variable is supported in C, its value
493is returned via $? if any of the C<gethost*()> functions fail.
494
19799a22 495If you have installed a signal handler for C<SIGCHLD>, the
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496value of C<$?> will usually be wrong outside that handler.
497
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498Inside an C<END> subroutine C<$?> contains the value that is going to be
499given to C<exit()>. You can modify C<$?> in an C<END> subroutine to
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500change the exit status of your program. For example:
501
502 END {
503 $? = 1 if $? == 255; # die would make it 255
504 }
a8f8344d 505
aa689395 506Under VMS, the pragma C<use vmsish 'status'> makes C<$?> reflect the
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507actual VMS exit status, instead of the default emulation of POSIX
508status.
f86702cc 509
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510Also see L<Error Indicators>.
511
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512=item $OS_ERROR
513
514=item $ERRNO
515
516=item $!
517
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518If used numerically, yields the current value of the C C<errno>
519variable, with all the usual caveats. (This means that you shouldn't
520depend on the value of C<$!> to be anything in particular unless
521you've gotten a specific error return indicating a system error.)
522If used an a string, yields the corresponding system error string.
523You can assign a number to C<$!> to set I<errno> if, for instance,
524you want C<"$!"> to return the string for error I<n>, or you want
525to set the exit value for the die() operator. (Mnemonic: What just
526went bang?)
a0d0e21e 527
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528Also see L<Error Indicators>.
529
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530=item $EXTENDED_OS_ERROR
531
532=item $^E
533
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534Error information specific to the current operating system. At
535the moment, this differs from C<$!> under only VMS, OS/2, and Win32
536(and for MacPerl). On all other platforms, C<$^E> is always just
537the same as C<$!>.
538
539Under VMS, C<$^E> provides the VMS status value from the last
540system error. This is more specific information about the last
541system error than that provided by C<$!>. This is particularly
d516a115 542important when C<$!> is set to B<EVMSERR>.
22fae026 543
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544Under OS/2, C<$^E> is set to the error code of the last call to
545OS/2 API either via CRT, or directly from perl.
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546
547Under Win32, C<$^E> always returns the last error information
548reported by the Win32 call C<GetLastError()> which describes
549the last error from within the Win32 API. Most Win32-specific
19799a22 550code will report errors via C<$^E>. ANSI C and Unix-like calls
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551set C<errno> and so most portable Perl code will report errors
552via C<$!>.
553
554Caveats mentioned in the description of C<$!> generally apply to
555C<$^E>, also. (Mnemonic: Extra error explanation.)
5c055ba3 556
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557Also see L<Error Indicators>.
558
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559=item $EVAL_ERROR
560
561=item $@
562
19799a22 563The Perl syntax error message from the last eval() operator. If null, the
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564last eval() parsed and executed correctly (although the operations you
565invoked may have failed in the normal fashion). (Mnemonic: Where was
566the syntax error "at"?)
567
19799a22 568Warning messages are not collected in this variable. You can,
a8f8344d 569however, set up a routine to process warnings by setting C<$SIG{__WARN__}>
54310121 570as described below.
748a9306 571
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572Also see L<Error Indicators>.
573
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574=item $PROCESS_ID
575
576=item $PID
577
578=item $$
579
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580The process number of the Perl running this script. You should
581consider this variable read-only, although it will be altered
582across fork() calls. (Mnemonic: same as shells.)
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583
584=item $REAL_USER_ID
585
586=item $UID
587
588=item $<
589
19799a22 590The real uid of this process. (Mnemonic: it's the uid you came I<from>,
a0d0e21e
LW
591if you're running setuid.)
592
593=item $EFFECTIVE_USER_ID
594
595=item $EUID
596
597=item $>
598
599The effective uid of this process. Example:
600
601 $< = $>; # set real to effective uid
602 ($<,$>) = ($>,$<); # swap real and effective uid
603
19799a22
GS
604(Mnemonic: it's the uid you went I<to>, if you're running setuid.)
605Note: C<$E<lt>> and C<$E<gt>> can be swapped only on machines
8cc95fdb 606supporting setreuid().
a0d0e21e
LW
607
608=item $REAL_GROUP_ID
609
610=item $GID
611
612=item $(
613
614The real gid of this process. If you are on a machine that supports
615membership in multiple groups simultaneously, gives a space separated
616list of groups you are in. The first number is the one returned by
617getgid(), and the subsequent ones by getgroups(), one of which may be
8cc95fdb
PP
618the same as the first number.
619
19799a22
GS
620However, a value assigned to C<$(> must be a single number used to
621set the real gid. So the value given by C<$(> should I<not> be assigned
622back to C<$(> without being forced numeric, such as by adding zero.
8cc95fdb 623
19799a22
GS
624(Mnemonic: parentheses are used to I<group> things. The real gid is the
625group you I<left>, if you're running setgid.)
a0d0e21e
LW
626
627=item $EFFECTIVE_GROUP_ID
628
629=item $EGID
630
631=item $)
632
633The effective gid of this process. If you are on a machine that
634supports membership in multiple groups simultaneously, gives a space
635separated list of groups you are in. The first number is the one
636returned by getegid(), and the subsequent ones by getgroups(), one of
8cc95fdb
PP
637which may be the same as the first number.
638
19799a22 639Similarly, a value assigned to C<$)> must also be a space-separated
8cc95fdb
PP
640list of numbers. The first number is used to set the effective gid, and
641the rest (if any) are passed to setgroups(). To get the effect of an
642empty list for setgroups(), just repeat the new effective gid; that is,
643to force an effective gid of 5 and an effectively empty setgroups()
644list, say C< $) = "5 5" >.
645
19799a22
GS
646(Mnemonic: parentheses are used to I<group> things. The effective gid
647is the group that's I<right> for you, if you're running setgid.)
a0d0e21e 648
19799a22
GS
649Note: C<$E<lt>>, C<$E<gt>>, C<$(> and C<$)> can be set only on
650machines that support the corresponding I<set[re][ug]id()> routine. C<$(>
651and C<$)> can be swapped only on machines supporting setregid().
a0d0e21e
LW
652
653=item $PROGRAM_NAME
654
655=item $0
656
19799a22
GS
657Contains the name of the program being executed. On some operating
658systems assigning to C<$0> modifies the argument area that the B<ps>
659program sees. This is more useful as a way of indicating the current
660program state than it is for hiding the program you're running.
a0d0e21e
LW
661(Mnemonic: same as B<sh> and B<ksh>.)
662
663=item $[
664
665The index of the first element in an array, and of the first character
19799a22
GS
666in a substring. Default is 0, but you could theoretically set it
667to 1 to make Perl behave more like B<awk> (or Fortran) when
668subscripting and when evaluating the index() and substr() functions.
669(Mnemonic: [ begins subscripts.)
a0d0e21e 670
19799a22
GS
671As of release 5 of Perl, assignment to C<$[> is treated as a compiler
672directive, and cannot influence the behavior of any other file.
673Its use is highly discouraged.
a0d0e21e
LW
674
675=item $PERL_VERSION
676
677=item $]
678
54310121
PP
679The version + patchlevel / 1000 of the Perl interpreter. This variable
680can be used to determine whether the Perl interpreter executing a
681script is in the right range of versions. (Mnemonic: Is this version
682of perl in the right bracket?) Example:
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LW
683
684 warn "No checksumming!\n" if $] < 3.019;
685
54310121 686See also the documentation of C<use VERSION> and C<require VERSION>
19799a22 687for a convenient way to fail if the running Perl interpreter is too old.
a0d0e21e 688
305aace0
NIS
689=item $COMPILING
690
691=item $^C
692
19799a22
GS
693The current value of the flag associated with the B<-c> switch.
694Mainly of use with B<-MO=...> to allow code to alter its behavior
695when being compiled, such as for example to AUTOLOAD at compile
696time rather than normal, deferred loading. See L<perlcc>. Setting
697C<$^C = 1> is similar to calling C<B::minus_c>.
305aace0 698
a0d0e21e
LW
699=item $DEBUGGING
700
701=item $^D
702
703The current value of the debugging flags. (Mnemonic: value of B<-D>
704switch.)
705
706=item $SYSTEM_FD_MAX
707
708=item $^F
709
710The maximum system file descriptor, ordinarily 2. System file
711descriptors are passed to exec()ed processes, while higher file
712descriptors are not. Also, during an open(), system file descriptors are
713preserved even if the open() fails. (Ordinary file descriptors are
19799a22 714closed before the open() is attempted.) The close-on-exec
a0d0e21e 715status of a file descriptor will be decided according to the value of
4771b018 716C<$^F> when the open() or pipe() was called, not the time of the exec().
a0d0e21e 717
6e2995f4
PP
718=item $^H
719
fb73857a
PP
720The current set of syntax checks enabled by C<use strict> and other block
721scoped compiler hints. See the documentation of C<strict> for more details.
6e2995f4 722
a0d0e21e
LW
723=item $INPLACE_EDIT
724
725=item $^I
726
727The current value of the inplace-edit extension. Use C<undef> to disable
728inplace editing. (Mnemonic: value of B<-i> switch.)
729
fb73857a
PP
730=item $^M
731
19799a22
GS
732By default, running out of memory is an untrappable, fatal error.
733However, if suitably built, Perl can use the contents of C<$^M>
734as an emergency memory pool after die()ing. Suppose that your Perl
735were compiled with -DPERL_EMERGENCY_SBRK and used Perl's malloc.
736Then
fb73857a 737
19799a22 738 $^M = 'a' x (1 << 16);
fb73857a 739
19799a22
GS
740would allocate a 64K buffer for use when in emergency. See the
741F<INSTALL> file in the Perl distribution for information on how to
742enable this option. To discourage casual use of this advanced
743feature, there is no L<English> long name for this variable.
fb73857a 744
5c055ba3 745=item $OSNAME
6e2995f4 746
5c055ba3
PP
747=item $^O
748
749The name of the operating system under which this copy of Perl was
750built, as determined during the configuration process. The value
19799a22
GS
751is identical to C<$Config{'osname'}>. See also L<Config> and the
752B<-V> command-line switch documented in L<perlrun>.
5c055ba3 753
a0d0e21e
LW
754=item $PERLDB
755
756=item $^P
757
19799a22
GS
758The internal variable for debugging support. The meanings of the
759various bits are subject to change, but currently indicate:
84902520
TB
760
761=over 6
762
763=item 0x01
764
765Debug subroutine enter/exit.
766
767=item 0x02
768
769Line-by-line debugging.
770
771=item 0x04
772
773Switch off optimizations.
774
775=item 0x08
776
777Preserve more data for future interactive inspections.
778
779=item 0x10
780
781Keep info about source lines on which a subroutine is defined.
782
783=item 0x20
784
785Start with single-step on.
786
787=back
788
19799a22
GS
789Some bits may be relevant at compile-time only, some at
790run-time only. This is a new mechanism and the details may change.
a0d0e21e 791
b9ac3b5b
GS
792=item $^R
793
19799a22
GS
794The result of evaluation of the last successful C<(?{ code })>
795regular expression assertion (see L<perlre>). May be written to.
b9ac3b5b 796
fb73857a
PP
797=item $^S
798
799Current state of the interpreter. Undefined if parsing of the current
800module/eval is not finished (may happen in $SIG{__DIE__} and
19799a22 801$SIG{__WARN__} handlers). True if inside an eval(), otherwise false.
fb73857a 802
a0d0e21e
LW
803=item $BASETIME
804
805=item $^T
806
19799a22 807The time at which the program began running, in seconds since the
5f05dabc 808epoch (beginning of 1970). The values returned by the B<-M>, B<-A>,
19799a22 809and B<-C> filetests are based on this value.
a0d0e21e
LW
810
811=item $WARNING
812
813=item $^W
814
19799a22
GS
815The current value of the warning switch, initially true if B<-w>
816was used, false otherwise, but directly modifiable. (Mnemonic:
817related to the B<-w> switch.) See also L<warning>.
a0d0e21e
LW
818
819=item $EXECUTABLE_NAME
820
821=item $^X
822
823The name that the Perl binary itself was executed as, from C's C<argv[0]>.
19799a22 824This may not be a full pathname, nor even necessarily in your path.
a0d0e21e
LW
825
826=item $ARGV
827
a8f8344d 828contains the name of the current file when reading from E<lt>E<gt>.
a0d0e21e
LW
829
830=item @ARGV
831
19799a22
GS
832The array @ARGV contains the command-line arguments intended for
833the script. C<$#ARGV> is the generally number of arguments minus
834one, because C<$ARGV[0]> is the first argument, I<not> the program's
835command name itself. See C<$0> for the command name.
a0d0e21e
LW
836
837=item @INC
838
19799a22
GS
839The array @INC contains the list of places that the C<do EXPR>,
840C<require>, or C<use> constructs look for their library files. It
841initially consists of the arguments to any B<-I> command-line
842switches, followed by the default Perl library, probably
843F</usr/local/lib/perl>, followed by ".", to represent the current
844directory. If you need to modify this at runtime, you should use
845the C<use lib> pragma to get the machine-dependent library properly
846loaded also:
a0d0e21e 847
cb1a09d0
AD
848 use lib '/mypath/libdir/';
849 use SomeMod;
303f2f76 850
fb73857a
PP
851=item @_
852
853Within a subroutine the array @_ contains the parameters passed to that
19799a22 854subroutine. See L<perlsub>.
fb73857a 855
a0d0e21e
LW
856=item %INC
857
19799a22
GS
858The hash %INC contains entries for each filename included via the
859C<do>, C<require>, or C<use> operators. The key is the filename
860you specified (with module names converted to pathnames), and the
861value is the location of the file actually found. The C<require>
862operator uses this array to determine whether a particular file has
863already been included.
a0d0e21e 864
b687b08b
TC
865=item %ENV
866
867=item $ENV{expr}
a0d0e21e
LW
868
869The hash %ENV contains your current environment. Setting a
19799a22
GS
870value in C<ENV> changes the environment for any child processes
871you subsequently fork() off.
a0d0e21e 872
b687b08b
TC
873=item %SIG
874
875=item $SIG{expr}
a0d0e21e
LW
876
877The hash %SIG is used to set signal handlers for various
19799a22 878signals. For example:
a0d0e21e
LW
879
880 sub handler { # 1st argument is signal name
fb73857a 881 my($sig) = @_;
a0d0e21e
LW
882 print "Caught a SIG$sig--shutting down\n";
883 close(LOG);
884 exit(0);
885 }
886
fb73857a
PP
887 $SIG{'INT'} = \&handler;
888 $SIG{'QUIT'} = \&handler;
a0d0e21e 889 ...
19799a22 890 $SIG{'INT'} = 'DEFAULT'; # restore default action
a0d0e21e
LW
891 $SIG{'QUIT'} = 'IGNORE'; # ignore SIGQUIT
892
f648820c
GS
893Using a value of C<'IGNORE'> usually has the effect of ignoring the
894signal, except for the C<CHLD> signal. See L<perlipc> for more about
895this special case.
896
19799a22 897Here are some other examples:
a0d0e21e 898
fb73857a 899 $SIG{"PIPE"} = "Plumber"; # assumes main::Plumber (not recommended)
a0d0e21e 900 $SIG{"PIPE"} = \&Plumber; # just fine; assume current Plumber
19799a22 901 $SIG{"PIPE"} = *Plumber; # somewhat esoteric
a0d0e21e
LW
902 $SIG{"PIPE"} = Plumber(); # oops, what did Plumber() return??
903
19799a22
GS
904Be sure not to use a bareword as the name of a signal handler,
905lest you inadvertently call it.
748a9306 906
44a8e56a
PP
907If your system has the sigaction() function then signal handlers are
908installed using it. This means you get reliable signal handling. If
909your system has the SA_RESTART flag it is used when signals handlers are
19799a22 910installed. This means that system calls for which restarting is supported
44a8e56a
PP
911continue rather than returning when a signal arrives. If you want your
912system calls to be interrupted by signal delivery then do something like
913this:
914
915 use POSIX ':signal_h';
916
917 my $alarm = 0;
918 sigaction SIGALRM, new POSIX::SigAction sub { $alarm = 1 }
919 or die "Error setting SIGALRM handler: $!\n";
920
921See L<POSIX>.
922
748a9306 923Certain internal hooks can be also set using the %SIG hash. The
a8f8344d 924routine indicated by C<$SIG{__WARN__}> is called when a warning message is
748a9306
LW
925about to be printed. The warning message is passed as the first
926argument. The presence of a __WARN__ hook causes the ordinary printing
927of warnings to STDERR to be suppressed. You can use this to save warnings
928in a variable, or turn warnings into fatal errors, like this:
929
930 local $SIG{__WARN__} = sub { die $_[0] };
931 eval $proggie;
932
a8f8344d 933The routine indicated by C<$SIG{__DIE__}> is called when a fatal exception
748a9306
LW
934is about to be thrown. The error message is passed as the first
935argument. When a __DIE__ hook routine returns, the exception
936processing continues as it would have in the absence of the hook,
cb1a09d0 937unless the hook routine itself exits via a C<goto>, a loop exit, or a die().
774d564b 938The C<__DIE__> handler is explicitly disabled during the call, so that you
fb73857a
PP
939can die from a C<__DIE__> handler. Similarly for C<__WARN__>.
940
19799a22
GS
941Due to an implementation glitch, the C<$SIG{__DIE__}> hook is called
942even inside an eval(). Do not use this to rewrite a pending exception
943in C<$@>, or as a bizarre substitute for overriding CORE::GLOBAL::die().
944This strange action at a distance may be fixed in a future release
945so that C<$SIG{__DIE__}> is only called if your program is about
946to exit, as was the original intent. Any other use is deprecated.
947
948C<__DIE__>/C<__WARN__> handlers are very special in one respect:
949they may be called to report (probable) errors found by the parser.
950In such a case the parser may be in inconsistent state, so any
951attempt to evaluate Perl code from such a handler will probably
952result in a segfault. This means that warnings or errors that
953result from parsing Perl should be used with extreme caution, like
954this:
fb73857a
PP
955
956 require Carp if defined $^S;
957 Carp::confess("Something wrong") if defined &Carp::confess;
958 die "Something wrong, but could not load Carp to give backtrace...
959 To see backtrace try starting Perl with -MCarp switch";
960
961Here the first line will load Carp I<unless> it is the parser who
962called the handler. The second line will print backtrace and die if
963Carp was available. The third line will be executed only if Carp was
964not available.
965
19799a22
GS
966See L<perlfunc/die>, L<perlfunc/warn>, L<perlfunc/eval>, and
967L<warning> for additional information.
68dc0745 968
a0d0e21e 969=back
55602bd2
IZ
970
971=head2 Error Indicators
972
19799a22
GS
973The variables C<$@>, C<$!>, C<$^E>, and C<$?> contain information
974about different types of error conditions that may appear during
975execution of a Perl program. The variables are shown ordered by
976the "distance" between the subsystem which reported the error and
977the Perl process. They correspond to errors detected by the Perl
978interpreter, C library, operating system, or an external program,
979respectively.
55602bd2
IZ
980
981To illustrate the differences between these variables, consider the
19799a22 982following Perl expression, which uses a single-quoted string:
55602bd2 983
19799a22
GS
984 eval q{
985 open PIPE, "/cdrom/install |";
986 @res = <PIPE>;
987 close PIPE or die "bad pipe: $?, $!";
988 };
55602bd2
IZ
989
990After execution of this statement all 4 variables may have been set.
991
19799a22
GS
992C<$@> is set if the string to be C<eval>-ed did not compile (this
993may happen if C<open> or C<close> were imported with bad prototypes),
994or if Perl code executed during evaluation die()d . In these cases
995the value of $@ is the compile error, or the argument to C<die>
996(which will interpolate C<$!> and C<$?>!). (See also L<Fatal>,
997though.)
998
999When the eval() expression above is executed, open(), C<<PIPEE<gt>>,
1000and C<close> are translated to calls in the C run-time library and
1001thence to the operating system kernel. C<$!> is set to the C library's
1002C<errno> if one of these calls fails.
1003
1004Under a few operating systems, C<$^E> may contain a more verbose
1005error indicator, such as in this case, "CDROM tray not closed."
1006Systems that do not support extended error mesages leave $C<$^E>
1007the same as C<$!>.
1008
1009Finally, C<$?> may be set to non-0 value if the external program
1010F</cdrom/install> fails. The upper eight bits reflect specific
1011error conditions encountered by the program (the program's exit()
1012value). The lower eight bits reflect mode of failure, like signal
1013death and core dump information See wait(2) for details. In
1014contrast to C<$!> and C<$^E>, which are set only if error condition
1015is detected, the variable C<$?> is set on each C<wait> or pipe
1016C<close>, overwriting the old value. This is more like C<$@>, which
1017on every eval() is always set on failure and cleared on success.
2b92dfce 1018
19799a22
GS
1019For more details, see the individual descriptions at C<$@>, C<$!>, C<$^E>,
1020and C<$?>.
2b92dfce
GS
1021
1022=head2 Technical Note on the Syntax of Variable Names
1023
19799a22
GS
1024Variable names in Perl can have several formats. Usually, they
1025must begin with a letter or underscore, in which case they can be
1026arbitrarily long (up to an internal limit of 251 characters) and
1027may contain letters, digits, underscores, or the special sequence
1028C<::> or C<'>. In this case, the part before the last C<::> or
1029C<'> is taken to be a I<package qualifier>; see L<perlmod>.
2b92dfce
GS
1030
1031Perl variable names may also be a sequence of digits or a single
1032punctuation or control character. These names are all reserved for
19799a22
GS
1033special uses by Perl; for example, the all-digits names are used
1034to hold data captured by backreferences after a regular expression
1035match. Perl has a special syntax for the single-control-character
1036names: It understands C<^X> (caret C<X>) to mean the control-C<X>
1037character. For example, the notation C<$^W> (dollar-sign caret
1038C<W>) is the scalar variable whose name is the single character
1039control-C<W>. This is better than typing a literal control-C<W>
1040into your program.
2b92dfce
GS
1041
1042Finally, new in Perl 5.006, Perl variable names may be alphanumeric
19799a22
GS
1043strings that begin with control characters (or better yet, a caret).
1044These variables must be written in the form C<${^Foo}>; the braces
1045are not optional. C<${^Foo}> denotes the scalar variable whose
1046name is a control-C<F> followed by two C<o>'s. These variables are
1047reserved for future special uses by Perl, except for the ones that
1048begin with C<^_> (control-underscore or caret-underscore). No
1049control-character name that begins with C<^_> will acquire a special
1050meaning in any future version of Perl; such names may therefore be
1051used safely in programs. C<$^_> itself, however, I<is> reserved.
1052
1053Perl identifiers that begin with digits, control characters, or
2b92dfce
GS
1054punctuation characters are exempt from the effects of the C<package>
1055declaration and are always forced to be in package C<main>. A few
1056other names are also exempt:
1057
1058 ENV STDIN
1059 INC STDOUT
1060 ARGV STDERR
1061 ARGVOUT
1062 SIG
1063
1064In particular, the new special C<${^_XYZ}> variables are always taken
19799a22 1065to be in package C<main>, regardless of any C<package> declarations
2b92dfce
GS
1066presently in scope.
1067
19799a22
GS
1068=head1 BUGS
1069
1070Due to an unfortunate accident of Perl's implementation, C<use
1071English> imposes a considerable performance penalty on all regular
1072expression matches in a program, regardless of whether they occur
1073in the scope of C<use English>. For that reason, saying C<use
1074English> in libraries is strongly discouraged. See the
1075Devel::SawAmpersand module documentation from CPAN
1076(http://www.perl.com/CPAN/modules/by-module/Devel/Devel-SawAmpersand-0.10.readme)
1077for more information.
2b92dfce 1078
19799a22
GS
1079Having to even think about the C<$^S> variable in your exception
1080handlers is simply wrong. C<$SIG{__DIE__}> as currently implemented
1081invites grievous and difficult to track down errors. Avoid it
1082and use an C<END{}> or CORE::GLOBAL::die override instead.