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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
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9runnable under C<use strict>. The third biggest trap is not reading
10the list of changes in this version of Perl; see L<perldelta>.
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11
12=head2 Awk Traps
13
14Accustomed B<awk> users should take special note of the following:
15
16=over 4
17
18=item *
19
20The English module, loaded via
21
22 use English;
23
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24allows you to refer to special variables (like C<$/>) with names (like
25C<$RS>), as though they were in B<awk>; see L<perlvar> for details.
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26
27=item *
28
29Semicolons are required after all simple statements in Perl (except
30at the end of a block). Newline is not a statement delimiter.
31
32=item *
33
34Curly brackets are required on C<if>s and C<while>s.
35
36=item *
37
5db417f7 38Variables begin with "$", "@" or "%" in Perl.
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39
40=item *
41
42Arrays index from 0. Likewise string positions in substr() and
43index().
44
45=item *
46
47You have to decide whether your array has numeric or string indices.
48
49=item *
50
aa689395 51Hash values do not spring into existence upon mere reference.
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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
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61to an array yourself. And the split() operator has different
62arguments than B<awk>'s.
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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
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104null string would render C</pat/ /pat/> unparsable, because the third slash
105would be interpreted as a division operator--the tokenizer 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 $RS to a pattern, only a string.
137
138=item *
139
140When in doubt, run the B<awk> construct through B<a2p> and see what it
141gives you.
142
143=back
144
145=head2 C Traps
146
147Cerebral C programmers should take note of the following:
148
149=over 4
150
151=item *
152
153Curly brackets are required on C<if>'s and C<while>'s.
154
155=item *
156
157You must use C<elsif> rather than C<else if>.
158
159=item *
160
54310121 161The C<break> and C<continue> keywords from C become in
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162Perl C<last> and C<next>, respectively.
163Unlike in C, these do I<NOT> work within a C<do { } while> construct.
164
165=item *
166
167There's no switch statement. (But it's easy to build one on the fly.)
168
169=item *
170
5db417f7 171Variables begin with "$", "@" or "%" in Perl.
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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
5f05dabc 186in Perl is the backslash, which creates a reference.
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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
54310121 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
54310121 239The backtick operator does no translation of the return value, unlike B<csh>.
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240
241=item *
242
243Shells (especially B<csh>) do several levels of substitution on each
5f05dabc 244command line. Perl does substitution in only certain constructs
54310121 245such as double quotes, backticks, angle brackets, and search patterns.
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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
68dc0745 277Avoid barewords if you can, especially all lowercase ones.
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278You can't tell by just looking at it whether a bareword is
279a function or a string. By using quotes on strings and
5f05dabc 280parentheses on function calls, you won't ever get them confused.
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281
282=item *
283
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284You cannot discern from mere inspection which builtins
285are unary operators (like chop() and chdir())
a0d0e21e 286and which are list operators (like print() and unlink()).
5f05dabc 287(User-defined subroutines can be B<only> list operators, never
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288unary ones.) See L<perlop>.
289
290=item *
291
748a9306 292People have a hard time remembering that some functions
a0d0e21e 293default to $_, or @ARGV, or whatever, but that others which
54310121 294you might expect to do not.
a0d0e21e 295
6dbacca0 296=item *
a0d0e21e 297
8b0a4b75 298The E<lt>FHE<gt> construct is not the name of the filehandle, it is a readline
5f05dabc 299operation on that handle. The data read is assigned to $_ only if the
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300file read is the sole condition in a while loop:
301
302 while (<FH>) { }
54310121 303 while (defined($_ = <FH>)) { }..
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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
54310121 316The C<do {}> construct isn't a real loop that you can use
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317loop control on.
318
319=item *
320
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321Use C<my()> for local variables whenever you can get away with
322it (but see L<perlform> for where you can't).
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
5f05dabc 335=head2 Perl4 to Perl5 Traps
a0d0e21e 336
54310121 337Practicing Perl4 Programmers should take note of the following
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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,
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395please submit it to Bill Middleton <F<wjm@best.com>> for inclusion.
396Also note that at least some of these can be caught with B<-w>.
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397
398=head2 Discontinuance, Deprecation, and BugFix traps
399
400Anything that has been discontinued, deprecated, or fixed as
54310121 401a bug from perl4.
a0d0e21e 402
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403=over 4
404
54310121 405=item * Discontinuance
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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";
54310121 415
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416 # perl4 prints: $_legacy is 1
417 # perl5 prints: $_legacy is
418
54310121 419=item * Deprecation
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420
421Double-colon is now a valid package separator in a variable name. Thus these
5f05dabc 422behave differently in perl4 vs. perl5, because the packages don't exist.
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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" ;
54310121 437
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438 # perl4 prints: x=10
439 # perl5 prints: Can't find string terminator "'" anywhere before EOF
a0d0e21e 440
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441You can avoid this problem, and remain compatible with perl4, if you
442always explicitly include the package name:
443
444 $x = 10 ;
445 print "x=${main'x}\n" ;
446
54310121 447Also see precedence traps, for parsing C<$:>.
a0d0e21e 448
6dbacca0 449=item * BugFix
a0d0e21e 450
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451The second and third arguments of C<splice()> are now evaluated in scalar
452context (as the Camel says) rather than list context.
a0d0e21e 453
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454 sub sub1{return(0,2) } # return a 2-elem array
455 sub sub2{ return(1,2,3)} # return a 3-elem array
54310121 456 @a1 = ("a","b","c","d","e");
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457 @a2 = splice(@a1,&sub1,&sub2);
458 print join(' ',@a2),"\n";
54310121 459
6dbacca0 460 # perl4 prints: a b
54310121 461 # perl5 prints: c d e
a0d0e21e 462
54310121 463=item * Discontinuance
a0d0e21e 464
6dbacca0 465You can't do a C<goto> into a block that is optimized away. Darn.
a0d0e21e 466
6dbacca0 467 goto marker1;
a0d0e21e 468
54310121 469 for(1){
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470 marker1:
471 print "Here I is!\n";
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472 }
473
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474 # perl4 prints: Here I is!
475 # perl5 dumps core (SEGV)
476
54310121 477=item * Discontinuance
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478
479It is no longer syntactically legal to use whitespace as the name
480of a variable, or as a delimiter for any kind of quote construct.
54310121 481Double darn.
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482
483 $a = ("foo bar");
484 $b = q baz ;
485 print "a is $a, b is $b\n";
54310121 486
6dbacca0 487 # perl4 prints: a is foo bar, b is baz
54310121 488 # perl5 errors: Bareword found where operator expected
5e378fdf 489
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490=item * Discontinuance
491
492The archaic while/if BLOCK BLOCK syntax is no longer supported.
493
494 if { 1 } {
495 print "True!";
496 }
497 else {
498 print "False!";
499 }
54310121 500
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501 # perl4 prints: True!
502 # perl5 errors: syntax error at test.pl line 1, near "if {"
503
504=item * BugFix
505
506The C<**> operator now binds more tightly than unary minus.
507It was documented to work this way before, but didn't.
508
509 print -4**2,"\n";
54310121 510
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511 # perl4 prints: 16
512 # perl5 prints: -16
513
54310121 514=item * Discontinuance
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515
516The meaning of C<foreach{}> has changed slightly when it is iterating over a
517list which is not an array. This used to assign the list to a
518temporary array, but no longer does so (for efficiency). This means
519that you'll now be iterating over the actual values, not over copies of
520the values. Modifications to the loop variable can change the original
521values.
522
523 @list = ('ab','abc','bcd','def');
524 foreach $var (grep(/ab/,@list)){
525 $var = 1;
526 }
527 print (join(':',@list));
54310121 528
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529 # perl4 prints: ab:abc:bcd:def
530 # perl5 prints: 1:1:bcd:def
531
532To retain Perl4 semantics you need to assign your list
54310121 533explicitly to a temporary array and then iterate over that. For
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534example, you might need to change
535
536 foreach $var (grep(/ab/,@list)){
537
538to
539
540 foreach $var (@tmp = grep(/ab/,@list)){
541
542Otherwise changing $var will clobber the values of @list. (This most often
543happens when you use C<$_> for the loop variable, and call subroutines in
544the loop that don't properly localize C<$_>.)
545
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546=item * Discontinuance
547
548C<split> with no arguments now behaves like C<split ' '> (which doesn't
549return an initial null field if $_ starts with whitespace), it used to
550behave like C<split /\s+/> (which does).
551
552 $_ = ' hi mom';
553 print join(':', split);
554
555 # perl4 prints: :hi:mom
556 # perl5 prints: hi:mom
557
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558=item * BugFix
559
9607fc9c 560Perl 4 would ignore any text which was attached to an B<-e> switch,
55497cff 561always taking the code snippet from the following arg. Additionally, it
9607fc9c 562would silently accept an B<-e> switch without a following arg. Both of
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563these behaviors have been fixed.
564
565 perl -e'print "attached to -e"' 'print "separate arg"'
54310121 566
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567 # perl4 prints: separate arg
568 # perl5 prints: attached to -e
54310121 569
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570 perl -e
571
572 # perl4 prints:
573 # perl5 dies: No code specified for -e.
574
575=item * Discontinuance
576
577In Perl 4 the return value of C<push> was undocumented, but it was
578actually the last value being pushed onto the target list. In Perl 5
579the return value of C<push> is documented, but has changed, it is the
580number of elements in the resulting list.
581
582 @x = ('existing');
583 print push(@x, 'first new', 'second new');
54310121 584
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585 # perl4 prints: second new
586 # perl5 prints: 3
587
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588=item * Discontinuance
589
590In Perl 4 (and versions of Perl 5 before 5.004), C<'\r'> characters in
591Perl code were silently allowed, although they could cause (mysterious!)
592failures in certain constructs, particularly here documents. Now,
593C<'\r'> characters cause an immediate fatal error. (Note: In this
594example, the notation B<\015> represents the incorrect line
595ending. Depending upon your text viewer, it will look different.)
596
597 print "foo";\015
598 print "bar";
599
600 # perl4 prints: foobar
601 # perl5.003 prints: foobar
602 # perl5.004 dies: Illegal character \015 (carriage return)
603
604See L<perldiag> for full details.
605
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606=item * Deprecation
607
608Some error messages will be different.
609
54310121 610=item * Discontinuance
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611
612Some bugs may have been inadvertently removed. :-)
613
614=back
615
616=head2 Parsing Traps
617
618Perl4-to-Perl5 traps from having to do with parsing.
619
620=over 4
621
622=item * Parsing
623
624Note the space between . and =
625
626 $string . = "more string";
627 print $string;
54310121 628
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629 # perl4 prints: more string
630 # perl5 prints: syntax error at - line 1, near ". ="
631
632=item * Parsing
633
634Better parsing in perl 5
635
636 sub foo {}
637 &foo
638 print("hello, world\n");
54310121 639
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640 # perl4 prints: hello, world
641 # perl5 prints: syntax error
642
643=item * Parsing
644
645"if it looks like a function, it is a function" rule.
646
647 print
648 ($foo == 1) ? "is one\n" : "is zero\n";
54310121 649
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650 # perl4 prints: is zero
651 # perl5 warns: "Useless use of a constant in void context" if using -w
652
653=back
654
655=head2 Numerical Traps
656
657Perl4-to-Perl5 traps having to do with numerical operators,
658operands, or output from same.
659
660=over 5
661
662=item * Numerical
663
664Formatted output and significant digits
665
666 print 7.373504 - 0, "\n";
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667 printf "%20.18f\n", 7.373504 - 0;
668
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669 # Perl4 prints:
670 7.375039999999996141
671 7.37503999999999614
54310121 672
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673 # Perl5 prints:
674 7.373504
675 7.37503999999999614
676
677=item * Numerical
678
5f05dabc 679This specific item has been deleted. It demonstrated how the auto-increment
5e378fdf 680operator would not catch when a number went over the signed int limit. Fixed
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681in version 5.003_04. But always be wary when using large integers.
682If in doubt:
6dbacca0 683
5e378fdf 684 use Math::BigInt;
6dbacca0 685
54310121 686=item * Numerical
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687
688Assignment of return values from numeric equality tests
689does not work in perl5 when the test evaluates to false (0).
690Logical tests now return an null, instead of 0
a6006777 691
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692 $p = ($test == 1);
693 print $p,"\n";
a6006777 694
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695 # perl4 prints: 0
696 # perl5 prints:
697
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698Also see L<"General Regular Expression Traps using s///, etc.">
699for another example of this new feature...
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700
701=back
702
703=head2 General data type traps
704
705Perl4-to-Perl5 traps involving most data-types, and their usage
706within certain expressions and/or context.
707
708=over 5
709
710=item * (Arrays)
711
712Negative array subscripts now count from the end of the array.
713
714 @a = (1, 2, 3, 4, 5);
715 print "The third element of the array is $a[3] also expressed as $a[-2] \n";
54310121 716
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717 # perl4 prints: The third element of the array is 4 also expressed as
718 # perl5 prints: The third element of the array is 4 also expressed as 4
719
720=item * (Arrays)
721
722Setting C<$#array> lower now discards array elements, and makes them
723impossible to recover.
724
54310121 725 @a = (a,b,c,d,e);
6dbacca0 726 print "Before: ",join('',@a);
54310121 727 $#a =1;
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728 print ", After: ",join('',@a);
729 $#a =3;
730 print ", Recovered: ",join('',@a),"\n";
54310121 731
6dbacca0
PP
732 # perl4 prints: Before: abcde, After: ab, Recovered: abcd
733 # perl5 prints: Before: abcde, After: ab, Recovered: ab
734
735=item * (Hashes)
736
737Hashes get defined before use
738
54310121 739 local($s,@a,%h);
6dbacca0
PP
740 die "scalar \$s defined" if defined($s);
741 die "array \@a defined" if defined(@a);
742 die "hash \%h defined" if defined(%h);
54310121 743
6dbacca0
PP
744 # perl4 prints:
745 # perl5 dies: hash %h defined
746
747=item * (Globs)
748
749glob assignment from variable to variable will fail if the assigned
750variable is localized subsequent to the assignment
751
752 @a = ("This is Perl 4");
753 *b = *a;
754 local(@a);
755 print @b,"\n";
54310121 756
6dbacca0
PP
757 # perl4 prints: This is Perl 4
758 # perl5 prints:
54310121 759
a3cb178b 760=item * (Globs)
54310121 761
a3cb178b
GS
762Assigning C<undef> to a glob has no effect in Perl 5. In Perl 4
763it undefines the associated scalar (but may have other side effects
764including SEGVs).
5e378fdf 765
6dbacca0
PP
766=item * (Scalar String)
767
768Changes in unary negation (of strings)
769This change effects both the return value and what it
770does to auto(magic)increment.
771
772 $x = "aaa";
773 print ++$x," : ";
774 print -$x," : ";
775 print ++$x,"\n";
54310121 776
6dbacca0
PP
777 # perl4 prints: aab : -0 : 1
778 # perl5 prints: aab : -aab : aac
779
780=item * (Constants)
781
782perl 4 lets you modify constants:
783
784 $foo = "x";
785 &mod($foo);
786 for ($x = 0; $x < 3; $x++) {
787 &mod("a");
788 }
789 sub mod {
790 print "before: $_[0]";
791 $_[0] = "m";
792 print " after: $_[0]\n";
793 }
54310121 794
6dbacca0
PP
795 # perl4:
796 # before: x after: m
797 # before: a after: m
798 # before: m after: m
799 # before: m after: m
54310121 800
6dbacca0
PP
801 # Perl5:
802 # before: x after: m
803 # Modification of a read-only value attempted at foo.pl line 12.
804 # before: a
805
806=item * (Scalars)
807
808The behavior is slightly different for:
809
810 print "$x", defined $x
54310121 811
6dbacca0
PP
812 # perl 4: 1
813 # perl 5: <no output, $x is not called into existence>
814
815=item * (Variable Suicide)
816
817Variable suicide behavior is more consistent under Perl 5.
aa689395 818Perl5 exhibits the same behavior for hashes and scalars,
5f05dabc 819that perl4 exhibits for only scalars.
6dbacca0
PP
820
821 $aGlobal{ "aKey" } = "global value";
822 print "MAIN:", $aGlobal{"aKey"}, "\n";
823 $GlobalLevel = 0;
824 &test( *aGlobal );
825
826 sub test {
827 local( *theArgument ) = @_;
828 local( %aNewLocal ); # perl 4 != 5.001l,m
54310121 829 $aNewLocal{"aKey"} = "this should never appear";
6dbacca0
PP
830 print "SUB: ", $theArgument{"aKey"}, "\n";
831 $aNewLocal{"aKey"} = "level $GlobalLevel"; # what should print
832 $GlobalLevel++;
833 if( $GlobalLevel<4 ) {
834 &test( *aNewLocal );
835 }
836 }
54310121 837
6dbacca0
PP
838 # Perl4:
839 # MAIN:global value
840 # SUB: global value
841 # SUB: level 0
842 # SUB: level 1
843 # SUB: level 2
54310121 844
6dbacca0
PP
845 # Perl5:
846 # MAIN:global value
847 # SUB: global value
848 # SUB: this should never appear
849 # SUB: this should never appear
850 # SUB: this should never appear
851
84dc3c4d 852=back
6dbacca0
PP
853
854=head2 Context Traps - scalar, list contexts
855
856=over 5
857
858=item * (list context)
859
860The elements of argument lists for formats are now evaluated in list
861context. This means you can interpolate list values now.
862
863 @fmt = ("foo","bar","baz");
864 format STDOUT=
865 @<<<<< @||||| @>>>>>
866 @fmt;
867 .
54310121
PP
868 write;
869
6dbacca0
PP
870 # perl4 errors: Please use commas to separate fields in file
871 # perl5 prints: foo bar baz
872
873=item * (scalar context)
874
54310121
PP
875The C<caller()> function now returns a false value in a scalar context
876if there is no caller. This lets library files determine if they're
6dbacca0
PP
877being required.
878
879 caller() ? (print "You rang?\n") : (print "Got a 0\n");
54310121 880
6dbacca0
PP
881 # perl4 errors: There is no caller
882 # perl5 prints: Got a 0
5e378fdf 883
6dbacca0
PP
884=item * (scalar context)
885
886The comma operator in a scalar context is now guaranteed to give a
887scalar context to its arguments.
888
889 @y= ('a','b','c');
890 $x = (1, 2, @y);
891 print "x = $x\n";
54310121 892
6dbacca0
PP
893 # Perl4 prints: x = c # Thinks list context interpolates list
894 # Perl5 prints: x = 3 # Knows scalar uses length of list
895
896=item * (list, builtin)
897
898C<sprintf()> funkiness (array argument converted to scalar array count)
899This test could be added to t/op/sprintf.t
900
901 @z = ('%s%s', 'foo', 'bar');
902 $x = sprintf(@z);
903 if ($x eq 'foobar') {print "ok 2\n";} else {print "not ok 2 '$x'\n";}
54310121 904
6dbacca0
PP
905 # perl4 prints: ok 2
906 # perl5 prints: not ok 2
907
908C<printf()> works fine, though:
909
910 printf STDOUT (@z);
54310121
PP
911 print "\n";
912
6dbacca0
PP
913 # perl4 prints: foobar
914 # perl5 prints: foobar
915
916Probably a bug.
917
918=back
919
920=head2 Precedence Traps
921
922Perl4-to-Perl5 traps involving precedence order.
923
84dc3c4d
PP
924=over 5
925
5e378fdf
PP
926=item * Precedence
927
8dbef698
JM
928LHS vs. RHS of any assignment operator. LHS is evaluated first
929in perl4, second in perl5; this can affect the relationship
930between side-effects in sub-expressions.
5e378fdf
PP
931
932 @arr = ( 'left', 'right' );
933 $a{shift @arr} = shift @arr;
934 print join( ' ', keys %a );
935
936 # perl4 prints: left
937 # perl5 prints: right
938
939=item * Precedence
6dbacca0
PP
940
941These are now semantic errors because of precedence:
942
943 @list = (1,2,3,4,5);
944 %map = ("a",1,"b",2,"c",3,"d",4);
945 $n = shift @list + 2; # first item in list plus 2
946 print "n is $n, ";
947 $m = keys %map + 2; # number of items in hash plus 2
948 print "m is $m\n";
54310121 949
6dbacca0
PP
950 # perl4 prints: n is 3, m is 6
951 # perl5 errors and fails to compile
952
953=item * Precedence
a0d0e21e 954
4633a7c4
LW
955The precedence of assignment operators is now the same as the precedence
956of assignment. Perl 4 mistakenly gave them the precedence of the associated
957operator. So you now must parenthesize them in expressions like
958
959 /foo/ ? ($a += 2) : ($a -= 2);
a6006777 960
4633a7c4
LW
961Otherwise
962
6dbacca0 963 /foo/ ? $a += 2 : $a -= 2
4633a7c4
LW
964
965would be erroneously parsed as
966
967 (/foo/ ? $a += 2 : $a) -= 2;
968
969On the other hand,
970
54310121 971 $a += /foo/ ? 1 : 2;
4633a7c4
LW
972
973now works as a C programmer would expect.
974
6dbacca0 975=item * Precedence
4633a7c4 976
6dbacca0 977 open FOO || die;
a0d0e21e 978
5f05dabc
PP
979is now incorrect. You need parentheses around the filehandle.
980Otherwise, perl5 leaves the statement as its default precedence:
a0d0e21e 981
6dbacca0 982 open(FOO || die);
54310121 983
6dbacca0
PP
984 # perl4 opens or dies
985 # perl5 errors: Precedence problem: open FOO should be open(FOO)
a0d0e21e 986
6dbacca0 987=item * Precedence
a0d0e21e 988
6dbacca0
PP
989perl4 gives the special variable, C<$:> precedence, where perl5
990treats C<$::> as main C<package>
a0d0e21e 991
6dbacca0 992 $a = "x"; print "$::a";
54310121 993
6dbacca0
PP
994 # perl 4 prints: -:a
995 # perl 5 prints: x
5e378fdf 996
6dbacca0 997=item * Precedence
a0d0e21e 998
54310121
PP
999concatenation precedence over filetest operator?
1000
1001 -e $foo .= "q"
a0d0e21e 1002
6dbacca0
PP
1003 # perl4 prints: no output
1004 # perl5 prints: Can't modify -e in concatenation
a0d0e21e 1005
6dbacca0
PP
1006=back
1007
1008=head2 General Regular Expression Traps using s///, etc.
1009
1010All types of RE traps.
1011
1012=over 5
1013
1014=item * Regular Expression
1015
1016C<s'$lhs'$rhs'> now does no interpolation on either side. It used to
54310121 1017interpolate C<$lhs> but not C<$rhs>. (And still does not match a literal
6dbacca0
PP
1018'$' in string)
1019
1020 $a=1;$b=2;
1021 $string = '1 2 $a $b';
1022 $string =~ s'$a'$b';
1023 print $string,"\n";
54310121 1024
6dbacca0
PP
1025 # perl4 prints: $b 2 $a $b
1026 # perl5 prints: 1 2 $a $b
1027
1028=item * Regular Expression
a0d0e21e
LW
1029
1030C<m//g> now attaches its state to the searched string rather than the
6dbacca0
PP
1031regular expression. (Once the scope of a block is left for the sub, the
1032state of the searched string is lost)
1033
1034 $_ = "ababab";
1035 while(m/ab/g){
1036 &doit("blah");
1037 }
1038 sub doit{local($_) = shift; print "Got $_ "}
54310121 1039
6dbacca0
PP
1040 # perl4 prints: blah blah blah
1041 # perl5 prints: infinite loop blah...
1042
1043=item * Regular Expression
1044
68dc0745
PP
1045Currently, if you use the C<m//o> qualifier on a regular expression
1046within an anonymous sub, I<all> closures generated from that anonymous
1047sub will use the regular expression as it was compiled when it was used
1048the very first time in any such closure. For instance, if you say
1049
1050 sub build_match {
1051 my($left,$right) = @_;
1052 return sub { $_[0] =~ /$left stuff $right/o; };
1053 }
1054
1055build_match() will always return a sub which matches the contents of
1056C<$left> and C<$right> as they were the I<first> time that build_match()
1057was called, not as they are in the current call.
1058
1059This is probably a bug, and may change in future versions of Perl.
1060
1061=item * Regular Expression
1062
6dbacca0
PP
1063If no parentheses are used in a match, Perl4 sets C<$+> to
1064the whole match, just like C<$&>. Perl5 does not.
1065
1066 "abcdef" =~ /b.*e/;
1067 print "\$+ = $+\n";
54310121 1068
6dbacca0
PP
1069 # perl4 prints: bcde
1070 # perl5 prints:
1071
1072=item * Regular Expression
1073
1074substitution now returns the null string if it fails
1075
1076 $string = "test";
1077 $value = ($string =~ s/foo//);
1078 print $value, "\n";
54310121 1079
6dbacca0
PP
1080 # perl4 prints: 0
1081 # perl5 prints:
1082
1083Also see L<Numerical Traps> for another example of this new feature.
1084
1085=item * Regular Expression
1086
54310121
PP
1087C<s`lhs`rhs`> (using backticks) is now a normal substitution, with no
1088backtick expansion
6dbacca0
PP
1089
1090 $string = "";
1091 $string =~ s`^`hostname`;
1092 print $string, "\n";
54310121 1093
6dbacca0
PP
1094 # perl4 prints: <the local hostname>
1095 # perl5 prints: hostname
1096
1097=item * Regular Expression
1098
1099Stricter parsing of variables used in regular expressions
1100
1101 s/^([^$grpc]*$grpc[$opt$plus$rep]?)//o;
54310121 1102
6dbacca0
PP
1103 # perl4: compiles w/o error
1104 # perl5: with Scalar found where operator expected ..., near "$opt$plus"
1105
1106an added component of this example, apparently from the same script, is
1107the actual value of the s'd string after the substitution.
1108C<[$opt]> is a character class in perl4 and an array subscript in perl5
1109
54310121 1110 $grpc = 'a';
6dbacca0
PP
1111 $opt = 'r';
1112 $_ = 'bar';
1113 s/^([^$grpc]*$grpc[$opt]?)/foo/;
1114 print ;
54310121 1115
6dbacca0
PP
1116 # perl4 prints: foo
1117 # perl5 prints: foobar
1118
1119=item * Regular Expression
1120
1121Under perl5, C<m?x?> matches only once, like C<?x?>. Under perl4, it matched
1122repeatedly, like C</x/> or C<m!x!>.
1123
1124 $test = "once";
1125 sub match { $test =~ m?once?; }
1126 &match();
1127 if( &match() ) {
1128 # m?x? matches more then once
1129 print "perl4\n";
54310121 1130 } else {
6dbacca0 1131 # m?x? matches only once
54310121 1132 print "perl5\n";
6dbacca0 1133 }
54310121 1134
6dbacca0
PP
1135 # perl4 prints: perl4
1136 # perl5 prints: perl5
a0d0e21e 1137
a0d0e21e 1138
44a8e56a
PP
1139=item * Regular Expression
1140
1141Under perl4 and upto version 5.003, a failed C<m//g> match used to
1142reset the internal iterator, so that subsequent C<m//g> match attempts
1143began from the beginning of the string. In perl version 5.004 and later,
1144failed C<m//g> matches do not reset the iterator position (which can be
1145found using the C<pos()> function--see L<perlfunc/pos>).
1146
1147 $test = "foop";
1148 for (1..3) {
1149 print $1 while ($test =~ /(o)/g);
1150 # pos $test = 0; # to get old behavior
1151 }
54310121 1152
44a8e56a
PP
1153 # perl4 prints: oooooo
1154 # perl5.004 prints: oo
1155
1156You may always reset the iterator yourself as shown in the commented line
1157to get the old behavior.
1158
6dbacca0
PP
1159=back
1160
1161=head2 Subroutine, Signal, Sorting Traps
a0d0e21e 1162
6dbacca0
PP
1163The general group of Perl4-to-Perl5 traps having to do with
1164Signals, Sorting, and their related subroutines, as well as
1165general subroutine traps. Includes some OS-Specific traps.
a0d0e21e 1166
6dbacca0 1167=over 5
a0d0e21e 1168
6dbacca0 1169=item * (Signals)
a0d0e21e 1170
6dbacca0
PP
1171Barewords that used to look like strings to Perl will now look like subroutine
1172calls if a subroutine by that name is defined before the compiler sees them.
a0d0e21e 1173
6dbacca0
PP
1174 sub SeeYa { warn"Hasta la vista, baby!" }
1175 $SIG{'TERM'} = SeeYa;
1176 print "SIGTERM is now $SIG{'TERM'}\n";
54310121 1177
6dbacca0
PP
1178 # perl4 prints: SIGTERM is main'SeeYa
1179 # perl5 prints: SIGTERM is now main::1
a0d0e21e 1180
6dbacca0 1181Use B<-w> to catch this one
a0d0e21e 1182
6dbacca0 1183=item * (Sort Subroutine)
a0d0e21e 1184
6dbacca0 1185reverse is no longer allowed as the name of a sort subroutine.
a0d0e21e 1186
6dbacca0 1187 sub reverse{ print "yup "; $a <=> $b }
54310121
PP
1188 print sort reverse a,b,c;
1189
6dbacca0 1190 # perl4 prints: yup yup yup yup abc
54310121 1191 # perl5 prints: abc
a0d0e21e 1192
b996531f
PP
1193=item * warn() won't let you specify a filehandle.
1194
1195Although it _always_ printed to STDERR, warn() would let you specify a
1196filehandle in perl4. With perl5 it does not.
5e378fdf
PP
1197
1198 warn STDERR "Foo!";
1199
1200 # perl4 prints: Foo!
54310121 1201 # perl5 prints: String found where operator expected
5e378fdf 1202
6dbacca0 1203=back
a0d0e21e 1204
6dbacca0
PP
1205=head2 OS Traps
1206
1207=over 5
1208
1209=item * (SysV)
1210
54310121
PP
1211Under HPUX, and some other SysV OSes, one had to reset any signal handler,
1212within the signal handler function, each time a signal was handled with
1213perl4. With perl5, the reset is now done correctly. Any code relying
6dbacca0
PP
1214on the handler _not_ being reset will have to be reworked.
1215
a6006777 1216Since version 5.002, Perl uses sigaction() under SysV.
6dbacca0
PP
1217
1218 sub gotit {
54310121
PP
1219 print "Got @_... ";
1220 }
6dbacca0 1221 $SIG{'INT'} = 'gotit';
54310121 1222
6dbacca0
PP
1223 $| = 1;
1224 $pid = fork;
1225 if ($pid) {
1226 kill('INT', $pid);
1227 sleep(1);
1228 kill('INT', $pid);
54310121 1229 } else {
6dbacca0 1230 while (1) {sleep(10);}
54310121
PP
1231 }
1232
6dbacca0
PP
1233 # perl4 (HPUX) prints: Got INT...
1234 # perl5 (HPUX) prints: Got INT... Got INT...
1235
1236=item * (SysV)
1237
54310121
PP
1238Under SysV OSes, C<seek()> on a file opened to append C<E<gt>E<gt>> now does
1239the right thing w.r.t. the fopen() manpage. e.g., - When a file is opened
6dbacca0
PP
1240for append, it is impossible to overwrite information already in
1241the file.
1242
1243 open(TEST,">>seek.test");
54310121 1244 $start = tell TEST ;
6dbacca0
PP
1245 foreach(1 .. 9){
1246 print TEST "$_ ";
1247 }
1248 $end = tell TEST ;
1249 seek(TEST,$start,0);
1250 print TEST "18 characters here";
54310121 1251
6dbacca0
PP
1252 # perl4 (solaris) seek.test has: 18 characters here
1253 # perl5 (solaris) seek.test has: 1 2 3 4 5 6 7 8 9 18 characters here
a0d0e21e 1254
a0d0e21e 1255
a0d0e21e 1256
6dbacca0 1257=back
a0d0e21e 1258
6dbacca0 1259=head2 Interpolation Traps
a0d0e21e 1260
8b0a4b75
PP
1261Perl4-to-Perl5 traps having to do with how things get interpolated
1262within certain expressions, statements, contexts, or whatever.
1263
6dbacca0 1264=over 5
a0d0e21e 1265
6dbacca0 1266=item * Interpolation
a0d0e21e 1267
6dbacca0
PP
1268@ now always interpolates an array in double-quotish strings.
1269
54310121
PP
1270 print "To: someone@somewhere.com\n";
1271
6dbacca0 1272 # perl4 prints: To:someone@somewhere.com
9607fc9c 1273 # perl5 errors : In string, @somewhere now must be written as \@somewhere
6dbacca0
PP
1274
1275=item * Interpolation
1276
6dbacca0
PP
1277Double-quoted strings may no longer end with an unescaped $ or @.
1278
1279 $foo = "foo$";
1280 $bar = "bar@";
1281 print "foo is $foo, bar is $bar\n";
54310121 1282
6dbacca0
PP
1283 # perl4 prints: foo is foo$, bar is bar@
1284 # perl5 errors: Final $ should be \$ or $name
1285
1286Note: perl5 DOES NOT error on the terminating @ in $bar
1287
1288=item * Interpolation
a0d0e21e 1289
8b0a4b75
PP
1290Perl now sometimes evaluates arbitrary expressions inside braces that occur
1291within double quotes (usually when the opening brace is preceded by C<$>
1292or C<@>).
1293
1294 @www = "buz";
1295 $foo = "foo";
1296 $bar = "bar";
1297 sub foo { return "bar" };
1298 print "|@{w.w.w}|${main'foo}|";
1299
1300 # perl4 prints: |@{w.w.w}|foo|
1301 # perl5 prints: |buz|bar|
1302
1303Note that you can C<use strict;> to ward off such trappiness under perl5.
1304
1305=item * Interpolation
1306
748a9306 1307The construct "this is $$x" used to interpolate the pid at that
6dbacca0 1308point, but now apparently tries to dereference C<$x>. C<$$> by itself still
748a9306
LW
1309works fine, however.
1310
6dbacca0 1311 print "this is $$x\n";
748a9306 1312
6dbacca0
PP
1313 # perl4 prints: this is XXXx (XXX is the current pid)
1314 # perl5 prints: this is
1315
1316=item * Interpolation
1317
54310121
PP
1318Creation of hashes on the fly with C<eval "EXPR"> now requires either both
1319C<$>'s to be protected in the specification of the hash name, or both curlies
6dbacca0
PP
1320to be protected. If both curlies are protected, the result will be compatible
1321with perl4 and perl5. This is a very common practice, and should be changed
1322to use the block form of C<eval{}> if possible.
c07a80fd 1323
6dbacca0
PP
1324 $hashname = "foobar";
1325 $key = "baz";
1326 $value = 1234;
1327 eval "\$$hashname{'$key'} = q|$value|";
1328 (defined($foobar{'baz'})) ? (print "Yup") : (print "Nope");
1329
1330 # perl4 prints: Yup
1331 # perl5 prints: Nope
1332
1333Changing
1334
1335 eval "\$$hashname{'$key'} = q|$value|";
c07a80fd
PP
1336
1337to
1338
6dbacca0 1339 eval "\$\$hashname{'$key'} = q|$value|";
c07a80fd 1340
6dbacca0 1341causes the following result:
c07a80fd 1342
6dbacca0
PP
1343 # perl4 prints: Nope
1344 # perl5 prints: Yup
c07a80fd 1345
6dbacca0 1346or, changing to
a0d0e21e 1347
6dbacca0
PP
1348 eval "\$$hashname\{'$key'\} = q|$value|";
1349
1350causes the following result:
1351
1352 # perl4 prints: Yup
1353 # perl5 prints: Yup
1354 # and is compatible for both versions
1355
1356
1357=item * Interpolation
1358
1359perl4 programs which unconsciously rely on the bugs in earlier perl versions.
1360
1361 perl -e '$bar=q/not/; print "This is $foo{$bar} perl5"'
54310121 1362
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1363 # perl4 prints: This is not perl5
1364 # perl5 prints: This is perl5
1365
1366=item * Interpolation
1367
54310121 1368You also have to be careful about array references.
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PP
1369
1370 print "$foo{"
1371
1372 perl 4 prints: {
1373 perl 5 prints: syntax error
1374
1375=item * Interpolation
1376
1377Similarly, watch out for:
1378
1379 $foo = "array";
1380 print "\$$foo{bar}\n";
54310121 1381
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1382 # perl4 prints: $array{bar}
1383 # perl5 prints: $
1384
1385Perl 5 is looking for C<$array{bar}> which doesn't exist, but perl 4 is
1386happy just to expand $foo to "array" by itself. Watch out for this
1387especially in C<eval>'s.
1388
1389=item * Interpolation
1390
1391C<qq()> string passed to C<eval>
1392
1393 eval qq(
1394 foreach \$y (keys %\$x\) {
1395 \$count++;
1396 }
1397 );
54310121 1398
6dbacca0 1399 # perl4 runs this ok
54310121 1400 # perl5 prints: Can't find string terminator ")"
a0d0e21e 1401
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1402=back
1403
1404=head2 DBM Traps
1405
1406General DBM traps.
1407
1408=over 5
1409
1410=item * DBM
1411
1412Existing dbm databases created under perl4 (or any other dbm/ndbm tool)
1413may cause the same script, run under perl5, to fail. The build of perl5
1414must have been linked with the same dbm/ndbm as the default for C<dbmopen()>
1415to function properly without C<tie>'ing to an extension dbm implementation.
1416
1417 dbmopen (%dbm, "file", undef);
1418 print "ok\n";
1419
1420 # perl4 prints: ok
1421 # perl5 prints: ok (IFF linked with -ldbm or -lndbm)
1422
1423
1424=item * DBM
1425
1426Existing dbm databases created under perl4 (or any other dbm/ndbm tool)
1427may cause the same script, run under perl5, to fail. The error generated
1428when exceeding the limit on the key/value size will cause perl5 to exit
1429immediately.
1430
1431 dbmopen(DB, "testdb",0600) || die "couldn't open db! $!";
1432 $DB{'trap'} = "x" x 1024; # value too large for most dbm/ndbm
1433 print "YUP\n";
1434
1435 # perl4 prints:
1436 dbm store returned -1, errno 28, key "trap" at - line 3.
1437 YUP
1438
1439 # perl5 prints:
1440 dbm store returned -1, errno 28, key "trap" at - line 3.
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LW
1441
1442=back
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1443
1444=head2 Unclassified Traps
1445
1446Everything else.
1447
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PP
1448=over 5
1449
5db417f7 1450=item * C<require>/C<do> trap using returned value
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1451
1452If the file doit.pl has:
1453
1454 sub foo {
1455 $rc = do "./do.pl";
1456 return 8;
54310121 1457 }
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1458 print &foo, "\n";
1459
1460And the do.pl file has the following single line:
1461
1462 return 3;
1463
1464Running doit.pl gives the following:
1465
1466 # perl 4 prints: 3 (aborts the subroutine early)
54310121 1467 # perl 5 prints: 8
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1468
1469Same behavior if you replace C<do> with C<require>.
1470
5db417f7
TB
1471=item * C<split> on empty string with LIMIT specified
1472
1473 $string = '';
1474 @list = split(/foo/, $string, 2)
1475
1476Perl4 returns a one element list containing the empty string but Perl5
1477returns an empty list.
1478
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PP
1479=back
1480
54310121 1481As always, if any of these are ever officially declared as bugs,
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1482they'll be fixed and removed.
1483