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1=head1 NAME
2
3perltrap - Perl traps for the unwary
4
5=head1 DESCRIPTION
6
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7The biggest trap of all is forgetting to use the B<-w> switch; see
8L<perlrun>. The second biggest trap is not making your entire program
9runnable under C<use strict>.
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10
11=head2 Awk Traps
12
13Accustomed B<awk> users should take special note of the following:
14
15=over 4
16
17=item *
18
19The English module, loaded via
20
21 use English;
22
23allows you to refer to special variables (like $RS) as
24though they were in B<awk>; see L<perlvar> for details.
25
26=item *
27
28Semicolons are required after all simple statements in Perl (except
29at the end of a block). Newline is not a statement delimiter.
30
31=item *
32
33Curly brackets are required on C<if>s and C<while>s.
34
35=item *
36
37Variables begin with "$" or "@" in Perl.
38
39=item *
40
41Arrays index from 0. Likewise string positions in substr() and
42index().
43
44=item *
45
46You have to decide whether your array has numeric or string indices.
47
48=item *
49
50Associative array values do not spring into existence upon mere
51reference.
52
53=item *
54
55You have to decide whether you want to use string or numeric
56comparisons.
57
58=item *
59
60Reading an input line does not split it for you. You get to split it
6dbacca0 61yourself to an array. And the split() operator has different
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62arguments.
63
64=item *
65
66The current input line is normally in $_, not $0. It generally does
67not have the newline stripped. ($0 is the name of the program
68executed.) See L<perlvar>.
69
70=item *
71
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72$E<lt>I<digit>E<gt> does not refer to fields--it refers to substrings matched
73by the last match pattern.
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74
75=item *
76
77The print() statement does not add field and record separators unless
8b0a4b75 78you set C<$,> and C<$\>. You can set $OFS and $ORS if you're using
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79the English module.
80
81=item *
82
83You must open your files before you print to them.
84
85=item *
86
87The range operator is "..", not comma. The comma operator works as in
88C.
89
90=item *
91
92The match operator is "=~", not "~". ("~" is the one's complement
93operator, as in C.)
94
95=item *
96
97The exponentiation operator is "**", not "^". "^" is the XOR
98operator, as in C. (You know, one could get the feeling that B<awk> is
99basically incompatible with C.)
100
101=item *
102
103The concatenation operator is ".", not the null string. (Using the
104null string would render C</pat/ /pat/> unparsable, since the third slash
105would be interpreted as a division operator--the tokener is in fact
8b0a4b75 106slightly context sensitive for operators like "/", "?", and "E<gt>".
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107And in fact, "." itself can be the beginning of a number.)
108
109=item *
110
111The C<next>, C<exit>, and C<continue> keywords work differently.
112
113=item *
114
115
116The following variables work differently:
117
118 Awk Perl
119 ARGC $#ARGV or scalar @ARGV
120 ARGV[0] $0
121 FILENAME $ARGV
122 FNR $. - something
123 FS (whatever you like)
124 NF $#Fld, or some such
125 NR $.
126 OFMT $#
127 OFS $,
128 ORS $\
129 RLENGTH length($&)
130 RS $/
131 RSTART length($`)
132 SUBSEP $;
133
134=item *
135
136You cannot set $RS to a pattern, only a string.
137
138=item *
139
140When in doubt, run the B<awk> construct through B<a2p> and see what it
141gives you.
142
143=back
144
145=head2 C Traps
146
147Cerebral C programmers should take note of the following:
148
149=over 4
150
151=item *
152
153Curly brackets are required on C<if>'s and C<while>'s.
154
155=item *
156
157You must use C<elsif> rather than C<else if>.
158
159=item *
160
161The C<break> and C<continue> keywords from C become in
162Perl C<last> and C<next>, respectively.
163Unlike in C, these do I<NOT> work within a C<do { } while> construct.
164
165=item *
166
167There's no switch statement. (But it's easy to build one on the fly.)
168
169=item *
170
171Variables begin with "$" or "@" in Perl.
172
173=item *
174
6dbacca0 175C<printf()> does not implement the "*" format for interpolating
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176field widths, but it's trivial to use interpolation of double-quoted
177strings to achieve the same effect.
178
179=item *
180
181Comments begin with "#", not "/*".
182
183=item *
184
185You can't take the address of anything, although a similar operator
186in Perl 5 is the backslash, which creates a reference.
187
188=item *
189
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190C<ARGV> must be capitalized. C<$ARGV[0]> is C's C<argv[1]>, and C<argv[0]>
191ends up in C<$0>.
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192
193=item *
194
195System calls such as link(), unlink(), rename(), etc. return nonzero for
196success, not 0.
197
198=item *
199
200Signal handlers deal with signal names, not numbers. Use C<kill -l>
201to find their names on your system.
202
203=back
204
205=head2 Sed Traps
206
207Seasoned B<sed> programmers should take note of the following:
208
209=over 4
210
211=item *
212
213Backreferences in substitutions use "$" rather than "\".
214
215=item *
216
217The pattern matching metacharacters "(", ")", and "|" do not have backslashes
218in front.
219
220=item *
221
222The range operator is C<...>, rather than comma.
223
224=back
225
226=head2 Shell Traps
227
228Sharp shell programmers should take note of the following:
229
230=over 4
231
232=item *
233
6dbacca0 234The backtick operator does variable interpolation without regard to
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235the presence of single quotes in the command.
236
237=item *
238
239The backtick operator does no translation of the return value, unlike B<csh>.
240
241=item *
242
243Shells (especially B<csh>) do several levels of substitution on each
244command line. Perl does substitution only in certain constructs
245such as double quotes, backticks, angle brackets, and search patterns.
246
247=item *
248
249Shells interpret scripts a little bit at a time. Perl compiles the
250entire program before executing it (except for C<BEGIN> blocks, which
251execute at compile time).
252
253=item *
254
255The arguments are available via @ARGV, not $1, $2, etc.
256
257=item *
258
259The environment is not automatically made available as separate scalar
260variables.
261
262=back
263
264=head2 Perl Traps
265
266Practicing Perl Programmers should take note of the following:
267
268=over 4
269
270=item *
271
272Remember that many operations behave differently in a list
273context than they do in a scalar one. See L<perldata> for details.
274
275=item *
276
277Avoid barewords if you can, especially all lower-case ones.
278You can't tell just by looking at it whether a bareword is
279a function or a string. By using quotes on strings and
280parens on function calls, you won't ever get them confused.
281
282=item *
283
284You cannot discern from mere inspection which built-ins
285are unary operators (like chop() and chdir())
286and which are list operators (like print() and unlink()).
287(User-defined subroutines can B<only> be list operators, never
288unary ones.) See L<perlop>.
289
290=item *
291
748a9306 292People have a hard time remembering that some functions
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293default to $_, or @ARGV, or whatever, but that others which
294you might expect to do not.
295
6dbacca0 296=item *
a0d0e21e 297
8b0a4b75 298The E<lt>FHE<gt> construct is not the name of the filehandle, it is a readline
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299operation on that handle. The data read is only assigned to $_ if the
300file read is the sole condition in a while loop:
301
302 while (<FH>) { }
303 while ($_ = <FH>) { }..
304 <FH>; # data discarded!
305
6dbacca0 306=item *
748a9306 307
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308Remember not to use "C<=>" when you need "C<=~>";
309these two constructs are quite different:
310
311 $x = /foo/;
312 $x =~ /foo/;
313
314=item *
315
316The C<do {}> construct isn't a real loop that you can use
317loop control on.
318
319=item *
320
6dbacca0 321Use C<my()> for local variables whenever you can get away with
a0d0e21e 322it (but see L<perlform> for where you can't).
6dbacca0 323Using C<local()> actually gives a local value to a global
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324variable, which leaves you open to unforeseen side-effects
325of dynamic scoping.
326
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327=item *
328
329If you localize an exported variable in a module, its exported value will
330not change. The local name becomes an alias to a new value but the
331external name is still an alias for the original.
332
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333=back
334
6dbacca0 335=head2 Perl4 to Perl5 Traps
a0d0e21e 336
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337Practicing Perl4 Programmers should take note of the following
338Perl4-to-Perl5 specific traps.
339
340They're crudely ordered according to the following list:
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341
342=over 4
343
6dbacca0 344=item Discontinuance, Deprecation, and BugFix traps
a0d0e21e 345
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346Anything that's been fixed as a perl4 bug, removed as a perl4 feature
347or deprecated as a perl4 feature with the intent to encourage usage of
348some other perl5 feature.
a0d0e21e 349
6dbacca0 350=item Parsing Traps
748a9306 351
6dbacca0 352Traps that appear to stem from the new parser.
a0d0e21e 353
6dbacca0 354=item Numerical Traps
a0d0e21e 355
6dbacca0 356Traps having to do with numerical or mathematical operators.
a0d0e21e 357
6dbacca0 358=item General data type traps
a0d0e21e 359
6dbacca0 360Traps involving perl standard data types.
a0d0e21e 361
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362=item Context Traps - scalar, list contexts
363
364Traps related to context within lists, scalar statements/declarations.
365
366=item Precedence Traps
367
368Traps related to the precedence of parsing, evaluation, and execution of
369code.
370
371=item General Regular Expression Traps using s///, etc.
372
373Traps related to the use of pattern matching.
374
375=item Subroutine, Signal, Sorting Traps
376
377Traps related to the use of signals and signal handlers, general subroutines,
378and sorting, along with sorting subroutines.
379
380=item OS Traps
381
382OS-specific traps.
383
384=item DBM Traps
385
386Traps specific to the use of C<dbmopen()>, and specific dbm implementations.
387
388=item Unclassified Traps
389
390Everything else.
391
392=back
393
394If you find an example of a conversion trap that is not listed here,
395please submit it to Bill Middleton F<wjm@best.com> for inclusion.
396Also note that at least some of these can be caught with C<-w>.
397
398=head2 Discontinuance, Deprecation, and BugFix traps
399
400Anything that has been discontinued, deprecated, or fixed as
401a bug from perl4.
a0d0e21e 402
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403=over 4
404
405=item * Discontinuance
406
407Symbols starting with "_" are no longer forced into package main, except
408for C<$_> itself (and C<@_>, etc.).
409
410 package test;
411 $_legacy = 1;
cb1a09d0 412
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413 package main;
414 print "\$_legacy is ",$_legacy,"\n";
415
416 # perl4 prints: $_legacy is 1
417 # perl5 prints: $_legacy is
418
419=item * Deprecation
420
421Double-colon is now a valid package separator in a variable name. Thus these
422behave differently in perl4 vs. perl5, since the packages don't exist.
423
424 $a=1;$b=2;$c=3;$var=4;
425 print "$a::$b::$c ";
cb1a09d0 426 print "$var::abc::xyz\n";
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427
428 # perl4 prints: 1::2::3 4::abc::xyz
429 # perl5 prints: 3
cb1a09d0 430
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431Given that C<::> is now the preferred package delimiter, it is debatable
432whether this should be classed as a bug or not.
433(The older package delimiter, ' ,is used here)
cb1a09d0 434
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435 $x = 10 ;
436 print "x=${'x}\n" ;
437
438 # perl4 prints: x=10
439 # perl5 prints: Can't find string terminator "'" anywhere before EOF
a0d0e21e 440
6dbacca0 441Also see precedence traps, for parsing C<$:>.
a0d0e21e 442
6dbacca0 443=item * BugFix
a0d0e21e 444
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445The second and third arguments of C<splice()> are now evaluated in scalar
446context (as the Camel says) rather than list context.
a0d0e21e 447
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448 sub sub1{return(0,2) } # return a 2-elem array
449 sub sub2{ return(1,2,3)} # return a 3-elem array
450 @a1 = ("a","b","c","d","e");
451 @a2 = splice(@a1,&sub1,&sub2);
452 print join(' ',@a2),"\n";
453
454 # perl4 prints: a b
455 # perl5 prints: c d e
a0d0e21e 456
6dbacca0 457=item * Discontinuance
a0d0e21e 458
6dbacca0 459You can't do a C<goto> into a block that is optimized away. Darn.
a0d0e21e 460
6dbacca0 461 goto marker1;
a0d0e21e 462
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463 for(1){
464 marker1:
465 print "Here I is!\n";
466 }
467
468 # perl4 prints: Here I is!
469 # perl5 dumps core (SEGV)
470
471=item * Discontinuance
472
473It is no longer syntactically legal to use whitespace as the name
474of a variable, or as a delimiter for any kind of quote construct.
475Double darn.
476
477 $a = ("foo bar");
478 $b = q baz ;
479 print "a is $a, b is $b\n";
480
481 # perl4 prints: a is foo bar, b is baz
482 # perl5 errors: Bare word found where operator expected
5e378fdf 483
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484=item * Discontinuance
485
486The archaic while/if BLOCK BLOCK syntax is no longer supported.
487
488 if { 1 } {
489 print "True!";
490 }
491 else {
492 print "False!";
493 }
494
495 # perl4 prints: True!
496 # perl5 errors: syntax error at test.pl line 1, near "if {"
497
498=item * BugFix
499
500The C<**> operator now binds more tightly than unary minus.
501It was documented to work this way before, but didn't.
502
503 print -4**2,"\n";
504
505 # perl4 prints: 16
506 # perl5 prints: -16
507
508=item * Discontinuance
509
510The meaning of C<foreach{}> has changed slightly when it is iterating over a
511list which is not an array. This used to assign the list to a
512temporary array, but no longer does so (for efficiency). This means
513that you'll now be iterating over the actual values, not over copies of
514the values. Modifications to the loop variable can change the original
515values.
516
517 @list = ('ab','abc','bcd','def');
518 foreach $var (grep(/ab/,@list)){
519 $var = 1;
520 }
521 print (join(':',@list));
522
523 # perl4 prints: ab:abc:bcd:def
524 # perl5 prints: 1:1:bcd:def
525
526To retain Perl4 semantics you need to assign your list
527explicitly to a temporary array and then iterate over that. For
528example, you might need to change
529
530 foreach $var (grep(/ab/,@list)){
531
532to
533
534 foreach $var (@tmp = grep(/ab/,@list)){
535
536Otherwise changing $var will clobber the values of @list. (This most often
537happens when you use C<$_> for the loop variable, and call subroutines in
538the loop that don't properly localize C<$_>.)
539
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540=item * Discontinuance
541
542C<split> with no arguments now behaves like C<split ' '> (which doesn't
543return an initial null field if $_ starts with whitespace), it used to
544behave like C<split /\s+/> (which does).
545
546 $_ = ' hi mom';
547 print join(':', split);
548
549 # perl4 prints: :hi:mom
550 # perl5 prints: hi:mom
551
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552=item * BugFix
553
554Perl 4 would ignore any text which was attached to an C<-e> switch,
555always taking the code snippet from the following arg. Additionally, it
556would silently accept an C<-e> switch without a following arg. Both of
557these behaviors have been fixed.
558
559 perl -e'print "attached to -e"' 'print "separate arg"'
560
561 # perl4 prints: separate arg
562 # perl5 prints: attached to -e
563
564 perl -e
565
566 # perl4 prints:
567 # perl5 dies: No code specified for -e.
568
569=item * Discontinuance
570
571In Perl 4 the return value of C<push> was undocumented, but it was
572actually the last value being pushed onto the target list. In Perl 5
573the return value of C<push> is documented, but has changed, it is the
574number of elements in the resulting list.
575
576 @x = ('existing');
577 print push(@x, 'first new', 'second new');
578
579 # perl4 prints: second new
580 # perl5 prints: 3
581
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582=item * Deprecation
583
584Some error messages will be different.
585
586=item * Discontinuance
587
588Some bugs may have been inadvertently removed. :-)
589
590=back
591
592=head2 Parsing Traps
593
594Perl4-to-Perl5 traps from having to do with parsing.
595
596=over 4
597
598=item * Parsing
599
600Note the space between . and =
601
602 $string . = "more string";
603 print $string;
604
605 # perl4 prints: more string
606 # perl5 prints: syntax error at - line 1, near ". ="
607
608=item * Parsing
609
610Better parsing in perl 5
611
612 sub foo {}
613 &foo
614 print("hello, world\n");
615
616 # perl4 prints: hello, world
617 # perl5 prints: syntax error
618
619=item * Parsing
620
621"if it looks like a function, it is a function" rule.
622
623 print
624 ($foo == 1) ? "is one\n" : "is zero\n";
625
626 # perl4 prints: is zero
627 # perl5 warns: "Useless use of a constant in void context" if using -w
628
629=back
630
631=head2 Numerical Traps
632
633Perl4-to-Perl5 traps having to do with numerical operators,
634operands, or output from same.
635
636=over 5
637
638=item * Numerical
639
640Formatted output and significant digits
641
642 print 7.373504 - 0, "\n";
643 printf "%20.18f\n", 7.373504 - 0;
644
645 # Perl4 prints:
646 7.375039999999996141
647 7.37503999999999614
648
649 # Perl5 prints:
650 7.373504
651 7.37503999999999614
652
653=item * Numerical
654
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655This specific item has been deleted. It demonstrated how the autoincrement
656operator would not catch when a number went over the signed int limit. Fixed
657in 5.003_04. But always be wary when using large ints. If in doubt:
6dbacca0 658
5e378fdf 659 use Math::BigInt;
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660
661=item * Numerical
662
663Assignment of return values from numeric equality tests
664does not work in perl5 when the test evaluates to false (0).
665Logical tests now return an null, instead of 0
666
667 $p = ($test == 1);
668 print $p,"\n";
669
670 # perl4 prints: 0
671 # perl5 prints:
672
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673Also see the L<General Regular Expression Traps using s///, etc.>
674tests for another example of this new feature...
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675
676=back
677
678=head2 General data type traps
679
680Perl4-to-Perl5 traps involving most data-types, and their usage
681within certain expressions and/or context.
682
683=over 5
684
685=item * (Arrays)
686
687Negative array subscripts now count from the end of the array.
688
689 @a = (1, 2, 3, 4, 5);
690 print "The third element of the array is $a[3] also expressed as $a[-2] \n";
691
692 # perl4 prints: The third element of the array is 4 also expressed as
693 # perl5 prints: The third element of the array is 4 also expressed as 4
694
695=item * (Arrays)
696
697Setting C<$#array> lower now discards array elements, and makes them
698impossible to recover.
699
700 @a = (a,b,c,d,e);
701 print "Before: ",join('',@a);
702 $#a =1;
703 print ", After: ",join('',@a);
704 $#a =3;
705 print ", Recovered: ",join('',@a),"\n";
706
707 # perl4 prints: Before: abcde, After: ab, Recovered: abcd
708 # perl5 prints: Before: abcde, After: ab, Recovered: ab
709
710=item * (Hashes)
711
712Hashes get defined before use
713
714 local($s,@a,%h);
715 die "scalar \$s defined" if defined($s);
716 die "array \@a defined" if defined(@a);
717 die "hash \%h defined" if defined(%h);
718
719 # perl4 prints:
720 # perl5 dies: hash %h defined
721
722=item * (Globs)
723
724glob assignment from variable to variable will fail if the assigned
725variable is localized subsequent to the assignment
726
727 @a = ("This is Perl 4");
728 *b = *a;
729 local(@a);
730 print @b,"\n";
731
732 # perl4 prints: This is Perl 4
733 # perl5 prints:
734
735 # Another example
736
737 *fred = *barney; # fred is aliased to barney
738 @barney = (1, 2, 4);
739 # @fred;
740 print "@fred"; # should print "1, 2, 4"
741
742 # perl4 prints: 1 2 4
743 # perl5 prints: Literal @fred now requires backslash
5e378fdf 744
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745=item * (Scalar String)
746
747Changes in unary negation (of strings)
748This change effects both the return value and what it
749does to auto(magic)increment.
750
751 $x = "aaa";
752 print ++$x," : ";
753 print -$x," : ";
754 print ++$x,"\n";
755
756 # perl4 prints: aab : -0 : 1
757 # perl5 prints: aab : -aab : aac
758
759=item * (Constants)
760
761perl 4 lets you modify constants:
762
763 $foo = "x";
764 &mod($foo);
765 for ($x = 0; $x < 3; $x++) {
766 &mod("a");
767 }
768 sub mod {
769 print "before: $_[0]";
770 $_[0] = "m";
771 print " after: $_[0]\n";
772 }
773
774 # perl4:
775 # before: x after: m
776 # before: a after: m
777 # before: m after: m
778 # before: m after: m
779
780 # Perl5:
781 # before: x after: m
782 # Modification of a read-only value attempted at foo.pl line 12.
783 # before: a
784
785=item * (Scalars)
786
787The behavior is slightly different for:
788
789 print "$x", defined $x
790
791 # perl 4: 1
792 # perl 5: <no output, $x is not called into existence>
793
794=item * (Variable Suicide)
795
796Variable suicide behavior is more consistent under Perl 5.
797Perl5 exhibits the same behavior for associative arrays and scalars,
798that perl4 exhibits only for scalars.
799
800 $aGlobal{ "aKey" } = "global value";
801 print "MAIN:", $aGlobal{"aKey"}, "\n";
802 $GlobalLevel = 0;
803 &test( *aGlobal );
804
805 sub test {
806 local( *theArgument ) = @_;
807 local( %aNewLocal ); # perl 4 != 5.001l,m
808 $aNewLocal{"aKey"} = "this should never appear";
809 print "SUB: ", $theArgument{"aKey"}, "\n";
810 $aNewLocal{"aKey"} = "level $GlobalLevel"; # what should print
811 $GlobalLevel++;
812 if( $GlobalLevel<4 ) {
813 &test( *aNewLocal );
814 }
815 }
816
817 # Perl4:
818 # MAIN:global value
819 # SUB: global value
820 # SUB: level 0
821 # SUB: level 1
822 # SUB: level 2
823
824 # Perl5:
825 # MAIN:global value
826 # SUB: global value
827 # SUB: this should never appear
828 # SUB: this should never appear
829 # SUB: this should never appear
830
84dc3c4d 831=back
6dbacca0
PP
832
833=head2 Context Traps - scalar, list contexts
834
835=over 5
836
837=item * (list context)
838
839The elements of argument lists for formats are now evaluated in list
840context. This means you can interpolate list values now.
841
842 @fmt = ("foo","bar","baz");
843 format STDOUT=
844 @<<<<< @||||| @>>>>>
845 @fmt;
846 .
847 write;
848
849 # perl4 errors: Please use commas to separate fields in file
850 # perl5 prints: foo bar baz
851
852=item * (scalar context)
853
854The C<caller()> function now returns a false value in a scalar context
855if there is no caller. This lets library files determine if they're
856being required.
857
858 caller() ? (print "You rang?\n") : (print "Got a 0\n");
859
860 # perl4 errors: There is no caller
861 # perl5 prints: Got a 0
5e378fdf 862
6dbacca0
PP
863=item * (scalar context)
864
865The comma operator in a scalar context is now guaranteed to give a
866scalar context to its arguments.
867
868 @y= ('a','b','c');
869 $x = (1, 2, @y);
870 print "x = $x\n";
871
872 # Perl4 prints: x = c # Thinks list context interpolates list
873 # Perl5 prints: x = 3 # Knows scalar uses length of list
874
875=item * (list, builtin)
876
877C<sprintf()> funkiness (array argument converted to scalar array count)
878This test could be added to t/op/sprintf.t
879
880 @z = ('%s%s', 'foo', 'bar');
881 $x = sprintf(@z);
882 if ($x eq 'foobar') {print "ok 2\n";} else {print "not ok 2 '$x'\n";}
883
884 # perl4 prints: ok 2
885 # perl5 prints: not ok 2
886
887C<printf()> works fine, though:
888
889 printf STDOUT (@z);
890 print "\n";
891
892 # perl4 prints: foobar
893 # perl5 prints: foobar
894
895Probably a bug.
896
897=back
898
899=head2 Precedence Traps
900
901Perl4-to-Perl5 traps involving precedence order.
902
84dc3c4d
PP
903=over 5
904
5e378fdf
PP
905=item * Precedence
906
907LHS vs. RHS when both sides are getting an op.
908
909 @arr = ( 'left', 'right' );
910 $a{shift @arr} = shift @arr;
911 print join( ' ', keys %a );
912
913 # perl4 prints: left
914 # perl5 prints: right
915
916=item * Precedence
6dbacca0
PP
917
918These are now semantic errors because of precedence:
919
920 @list = (1,2,3,4,5);
921 %map = ("a",1,"b",2,"c",3,"d",4);
922 $n = shift @list + 2; # first item in list plus 2
923 print "n is $n, ";
924 $m = keys %map + 2; # number of items in hash plus 2
925 print "m is $m\n";
926
927 # perl4 prints: n is 3, m is 6
928 # perl5 errors and fails to compile
929
930=item * Precedence
a0d0e21e 931
4633a7c4
LW
932The precedence of assignment operators is now the same as the precedence
933of assignment. Perl 4 mistakenly gave them the precedence of the associated
934operator. So you now must parenthesize them in expressions like
935
936 /foo/ ? ($a += 2) : ($a -= 2);
6dbacca0 937
4633a7c4
LW
938Otherwise
939
6dbacca0 940 /foo/ ? $a += 2 : $a -= 2
4633a7c4
LW
941
942would be erroneously parsed as
943
944 (/foo/ ? $a += 2 : $a) -= 2;
945
946On the other hand,
947
6dbacca0 948 $a += /foo/ ? 1 : 2;
4633a7c4
LW
949
950now works as a C programmer would expect.
951
6dbacca0 952=item * Precedence
4633a7c4 953
6dbacca0 954 open FOO || die;
a0d0e21e 955
6dbacca0
PP
956is now incorrect. You need parens around the filehandle.
957Otherwise, perl5 leaves the statement as it's default precedence:
a0d0e21e 958
6dbacca0
PP
959 open(FOO || die);
960
961 # perl4 opens or dies
962 # perl5 errors: Precedence problem: open FOO should be open(FOO)
a0d0e21e 963
6dbacca0 964=item * Precedence
a0d0e21e 965
6dbacca0
PP
966perl4 gives the special variable, C<$:> precedence, where perl5
967treats C<$::> as main C<package>
a0d0e21e 968
6dbacca0
PP
969 $a = "x"; print "$::a";
970
971 # perl 4 prints: -:a
972 # perl 5 prints: x
5e378fdf 973
6dbacca0 974=item * Precedence
a0d0e21e 975
6dbacca0 976concatenation precedence over filetest operator?
a0d0e21e 977
6dbacca0
PP
978 -e $foo .= "q"
979
980 # perl4 prints: no output
981 # perl5 prints: Can't modify -e in concatenation
a0d0e21e 982
6dbacca0 983=item * Precedence
a0d0e21e 984
6dbacca0
PP
985Assignment to value takes precedence over assignment to key in
986perl5 when using the shift operator on both sides.
987
988 @arr = ( 'left', 'right' );
989 $a{shift @arr} = shift @arr;
990 print join( ' ', keys %a );
991
992 # perl4 prints: left
993 # perl5 prints: right
994
995=back
996
997=head2 General Regular Expression Traps using s///, etc.
998
999All types of RE traps.
1000
1001=over 5
1002
1003=item * Regular Expression
1004
1005C<s'$lhs'$rhs'> now does no interpolation on either side. It used to
1006interpolate C<$lhs> but not C<$rhs>. (And still does not match a literal
1007'$' in string)
1008
1009 $a=1;$b=2;
1010 $string = '1 2 $a $b';
1011 $string =~ s'$a'$b';
1012 print $string,"\n";
1013
1014 # perl4 prints: $b 2 $a $b
1015 # perl5 prints: 1 2 $a $b
1016
1017=item * Regular Expression
a0d0e21e
LW
1018
1019C<m//g> now attaches its state to the searched string rather than the
6dbacca0
PP
1020regular expression. (Once the scope of a block is left for the sub, the
1021state of the searched string is lost)
1022
1023 $_ = "ababab";
1024 while(m/ab/g){
1025 &doit("blah");
1026 }
1027 sub doit{local($_) = shift; print "Got $_ "}
1028
1029 # perl4 prints: blah blah blah
1030 # perl5 prints: infinite loop blah...
1031
1032=item * Regular Expression
1033
1034If no parentheses are used in a match, Perl4 sets C<$+> to
1035the whole match, just like C<$&>. Perl5 does not.
1036
1037 "abcdef" =~ /b.*e/;
1038 print "\$+ = $+\n";
1039
1040 # perl4 prints: bcde
1041 # perl5 prints:
1042
1043=item * Regular Expression
1044
1045substitution now returns the null string if it fails
1046
1047 $string = "test";
1048 $value = ($string =~ s/foo//);
1049 print $value, "\n";
1050
1051 # perl4 prints: 0
1052 # perl5 prints:
1053
1054Also see L<Numerical Traps> for another example of this new feature.
1055
1056=item * Regular Expression
1057
1058C<s`lhs`rhs`> (using backticks) is now a normal substitution, with no
1059backtick expansion
1060
1061 $string = "";
1062 $string =~ s`^`hostname`;
1063 print $string, "\n";
1064
1065 # perl4 prints: <the local hostname>
1066 # perl5 prints: hostname
1067
1068=item * Regular Expression
1069
1070Stricter parsing of variables used in regular expressions
1071
1072 s/^([^$grpc]*$grpc[$opt$plus$rep]?)//o;
1073
1074 # perl4: compiles w/o error
1075 # perl5: with Scalar found where operator expected ..., near "$opt$plus"
1076
1077an added component of this example, apparently from the same script, is
1078the actual value of the s'd string after the substitution.
1079C<[$opt]> is a character class in perl4 and an array subscript in perl5
1080
1081 $grpc = 'a';
1082 $opt = 'r';
1083 $_ = 'bar';
1084 s/^([^$grpc]*$grpc[$opt]?)/foo/;
1085 print ;
1086
1087 # perl4 prints: foo
1088 # perl5 prints: foobar
1089
1090=item * Regular Expression
1091
1092Under perl5, C<m?x?> matches only once, like C<?x?>. Under perl4, it matched
1093repeatedly, like C</x/> or C<m!x!>.
1094
1095 $test = "once";
1096 sub match { $test =~ m?once?; }
1097 &match();
1098 if( &match() ) {
1099 # m?x? matches more then once
1100 print "perl4\n";
1101 } else {
1102 # m?x? matches only once
1103 print "perl5\n";
1104 }
1105
1106 # perl4 prints: perl4
1107 # perl5 prints: perl5
a0d0e21e 1108
a0d0e21e 1109
6dbacca0
PP
1110=back
1111
1112=head2 Subroutine, Signal, Sorting Traps
a0d0e21e 1113
6dbacca0
PP
1114The general group of Perl4-to-Perl5 traps having to do with
1115Signals, Sorting, and their related subroutines, as well as
1116general subroutine traps. Includes some OS-Specific traps.
a0d0e21e 1117
6dbacca0 1118=over 5
a0d0e21e 1119
6dbacca0 1120=item * (Signals)
a0d0e21e 1121
6dbacca0
PP
1122Barewords that used to look like strings to Perl will now look like subroutine
1123calls if a subroutine by that name is defined before the compiler sees them.
a0d0e21e 1124
6dbacca0
PP
1125 sub SeeYa { warn"Hasta la vista, baby!" }
1126 $SIG{'TERM'} = SeeYa;
1127 print "SIGTERM is now $SIG{'TERM'}\n";
1128
1129 # perl4 prints: SIGTERM is main'SeeYa
1130 # perl5 prints: SIGTERM is now main::1
a0d0e21e 1131
6dbacca0 1132Use B<-w> to catch this one
a0d0e21e 1133
6dbacca0 1134=item * (Sort Subroutine)
a0d0e21e 1135
6dbacca0 1136reverse is no longer allowed as the name of a sort subroutine.
a0d0e21e 1137
6dbacca0
PP
1138 sub reverse{ print "yup "; $a <=> $b }
1139 print sort reverse a,b,c;
1140
1141 # perl4 prints: yup yup yup yup abc
1142 # perl5 prints: abc
a0d0e21e 1143
b996531f
PP
1144=item * warn() won't let you specify a filehandle.
1145
1146Although it _always_ printed to STDERR, warn() would let you specify a
1147filehandle in perl4. With perl5 it does not.
5e378fdf
PP
1148
1149 warn STDERR "Foo!";
1150
1151 # perl4 prints: Foo!
1152 # perl5 prints: String found where operator expected
1153
6dbacca0 1154=back
a0d0e21e 1155
6dbacca0
PP
1156=head2 OS Traps
1157
1158=over 5
1159
1160=item * (SysV)
1161
1162Under HPUX, and some other SysV OS's, one had to reset any signal handler,
1163within the signal handler function, each time a signal was handled with
1164perl4. With perl5, the reset is now done correctly. Any code relying
1165on the handler _not_ being reset will have to be reworked.
1166
11675.002 and beyond uses sigaction() under SysV
1168
1169 sub gotit {
1170 print "Got @_... ";
1171 }
1172 $SIG{'INT'} = 'gotit';
1173
1174 $| = 1;
1175 $pid = fork;
1176 if ($pid) {
1177 kill('INT', $pid);
1178 sleep(1);
1179 kill('INT', $pid);
1180 } else {
1181 while (1) {sleep(10);}
1182 }
1183
1184 # perl4 (HPUX) prints: Got INT...
1185 # perl5 (HPUX) prints: Got INT... Got INT...
1186
1187=item * (SysV)
1188
1189Under SysV OS's, C<seek()> on a file opened to append C<E<gt>E<gt>> now does
1190the right thing w.r.t. the fopen() man page. e.g. - When a file is opened
1191for append, it is impossible to overwrite information already in
1192the file.
1193
1194 open(TEST,">>seek.test");
1195 $start = tell TEST ;
1196 foreach(1 .. 9){
1197 print TEST "$_ ";
1198 }
1199 $end = tell TEST ;
1200 seek(TEST,$start,0);
1201 print TEST "18 characters here";
1202
1203 # perl4 (solaris) seek.test has: 18 characters here
1204 # perl5 (solaris) seek.test has: 1 2 3 4 5 6 7 8 9 18 characters here
a0d0e21e 1205
a0d0e21e 1206
a0d0e21e 1207
6dbacca0 1208=back
a0d0e21e 1209
6dbacca0 1210=head2 Interpolation Traps
a0d0e21e 1211
8b0a4b75
PP
1212Perl4-to-Perl5 traps having to do with how things get interpolated
1213within certain expressions, statements, contexts, or whatever.
1214
6dbacca0 1215=over 5
a0d0e21e 1216
6dbacca0 1217=item * Interpolation
a0d0e21e 1218
6dbacca0
PP
1219@ now always interpolates an array in double-quotish strings.
1220
1221 print "To: someone@somewhere.com\n";
1222
1223 # perl4 prints: To:someone@somewhere.com
1224 # perl5 errors : Literal @somewhere now requires backslash
1225
1226=item * Interpolation
1227
6dbacca0
PP
1228Double-quoted strings may no longer end with an unescaped $ or @.
1229
1230 $foo = "foo$";
1231 $bar = "bar@";
1232 print "foo is $foo, bar is $bar\n";
1233
1234 # perl4 prints: foo is foo$, bar is bar@
1235 # perl5 errors: Final $ should be \$ or $name
1236
1237Note: perl5 DOES NOT error on the terminating @ in $bar
1238
1239=item * Interpolation
a0d0e21e 1240
8b0a4b75
PP
1241Perl now sometimes evaluates arbitrary expressions inside braces that occur
1242within double quotes (usually when the opening brace is preceded by C<$>
1243or C<@>).
1244
1245 @www = "buz";
1246 $foo = "foo";
1247 $bar = "bar";
1248 sub foo { return "bar" };
1249 print "|@{w.w.w}|${main'foo}|";
1250
1251 # perl4 prints: |@{w.w.w}|foo|
1252 # perl5 prints: |buz|bar|
1253
1254Note that you can C<use strict;> to ward off such trappiness under perl5.
1255
1256=item * Interpolation
1257
748a9306 1258The construct "this is $$x" used to interpolate the pid at that
6dbacca0 1259point, but now apparently tries to dereference C<$x>. C<$$> by itself still
748a9306
LW
1260works fine, however.
1261
6dbacca0 1262 print "this is $$x\n";
748a9306 1263
6dbacca0
PP
1264 # perl4 prints: this is XXXx (XXX is the current pid)
1265 # perl5 prints: this is
1266
1267=item * Interpolation
1268
1269Creation of hashes on the fly with C<eval "EXPR"> now requires either both
1270C<$>'s to be protected in the specification of the hash name, or both curlies
1271to be protected. If both curlies are protected, the result will be compatible
1272with perl4 and perl5. This is a very common practice, and should be changed
1273to use the block form of C<eval{}> if possible.
c07a80fd 1274
6dbacca0
PP
1275 $hashname = "foobar";
1276 $key = "baz";
1277 $value = 1234;
1278 eval "\$$hashname{'$key'} = q|$value|";
1279 (defined($foobar{'baz'})) ? (print "Yup") : (print "Nope");
1280
1281 # perl4 prints: Yup
1282 # perl5 prints: Nope
1283
1284Changing
1285
1286 eval "\$$hashname{'$key'} = q|$value|";
c07a80fd
PP
1287
1288to
1289
6dbacca0 1290 eval "\$\$hashname{'$key'} = q|$value|";
c07a80fd 1291
6dbacca0 1292causes the following result:
c07a80fd 1293
6dbacca0
PP
1294 # perl4 prints: Nope
1295 # perl5 prints: Yup
c07a80fd 1296
6dbacca0 1297or, changing to
a0d0e21e 1298
6dbacca0
PP
1299 eval "\$$hashname\{'$key'\} = q|$value|";
1300
1301causes the following result:
1302
1303 # perl4 prints: Yup
1304 # perl5 prints: Yup
1305 # and is compatible for both versions
1306
1307
1308=item * Interpolation
1309
1310perl4 programs which unconsciously rely on the bugs in earlier perl versions.
1311
1312 perl -e '$bar=q/not/; print "This is $foo{$bar} perl5"'
1313
1314 # perl4 prints: This is not perl5
1315 # perl5 prints: This is perl5
1316
1317=item * Interpolation
1318
1319You also have to be careful about array references.
1320
1321 print "$foo{"
1322
1323 perl 4 prints: {
1324 perl 5 prints: syntax error
1325
1326=item * Interpolation
1327
1328Similarly, watch out for:
1329
1330 $foo = "array";
1331 print "\$$foo{bar}\n";
1332
1333 # perl4 prints: $array{bar}
1334 # perl5 prints: $
1335
1336Perl 5 is looking for C<$array{bar}> which doesn't exist, but perl 4 is
1337happy just to expand $foo to "array" by itself. Watch out for this
1338especially in C<eval>'s.
1339
1340=item * Interpolation
1341
1342C<qq()> string passed to C<eval>
1343
1344 eval qq(
1345 foreach \$y (keys %\$x\) {
1346 \$count++;
1347 }
1348 );
1349
1350 # perl4 runs this ok
1351 # perl5 prints: Can't find string terminator ")"
a0d0e21e 1352
6dbacca0
PP
1353=back
1354
1355=head2 DBM Traps
1356
1357General DBM traps.
1358
1359=over 5
1360
1361=item * DBM
1362
1363Existing dbm databases created under perl4 (or any other dbm/ndbm tool)
1364may cause the same script, run under perl5, to fail. The build of perl5
1365must have been linked with the same dbm/ndbm as the default for C<dbmopen()>
1366to function properly without C<tie>'ing to an extension dbm implementation.
1367
1368 dbmopen (%dbm, "file", undef);
1369 print "ok\n";
1370
1371 # perl4 prints: ok
1372 # perl5 prints: ok (IFF linked with -ldbm or -lndbm)
1373
1374
1375=item * DBM
1376
1377Existing dbm databases created under perl4 (or any other dbm/ndbm tool)
1378may cause the same script, run under perl5, to fail. The error generated
1379when exceeding the limit on the key/value size will cause perl5 to exit
1380immediately.
1381
1382 dbmopen(DB, "testdb",0600) || die "couldn't open db! $!";
1383 $DB{'trap'} = "x" x 1024; # value too large for most dbm/ndbm
1384 print "YUP\n";
1385
1386 # perl4 prints:
1387 dbm store returned -1, errno 28, key "trap" at - line 3.
1388 YUP
1389
1390 # perl5 prints:
1391 dbm store returned -1, errno 28, key "trap" at - line 3.
a0d0e21e
LW
1392
1393=back
6dbacca0
PP
1394
1395=head2 Unclassified Traps
1396
1397Everything else.
1398
84dc3c4d
PP
1399=over 5
1400
6dbacca0
PP
1401=item * Unclassified
1402
1403C<require>/C<do> trap using returned value
1404
1405If the file doit.pl has:
1406
1407 sub foo {
1408 $rc = do "./do.pl";
1409 return 8;
1410 }
1411 print &foo, "\n";
1412
1413And the do.pl file has the following single line:
1414
1415 return 3;
1416
1417Running doit.pl gives the following:
1418
1419 # perl 4 prints: 3 (aborts the subroutine early)
1420 # perl 5 prints: 8
1421
1422Same behavior if you replace C<do> with C<require>.
1423
1424=back
1425
1426As always, if any of these are ever officially declared as bugs,
1427they'll be fixed and removed.
1428