3 perltrap - Perl traps for the unwary
7 The biggest trap of all is forgetting to use the B<-w> switch; see
8 L<perlrun>. The second biggest trap is not making your entire program
9 runnable under C<use strict>. The third biggest trap is not reading
10 the list of changes in this version of Perl; see L<perldelta>.
14 Accustomed B<awk> users should take special note of the following:
20 The English module, loaded via
24 allows you to refer to special variables (like C<$/>) with names (like
25 $RS), as though they were in B<awk>; see L<perlvar> for details.
29 Semicolons are required after all simple statements in Perl (except
30 at the end of a block). Newline is not a statement delimiter.
34 Curly brackets are required on C<if>s and C<while>s.
38 Variables begin with "$", "@" or "%" in Perl.
42 Arrays index from 0. Likewise string positions in substr() and
47 You have to decide whether your array has numeric or string indices.
51 Hash values do not spring into existence upon mere reference.
55 You have to decide whether you want to use string or numeric
60 Reading an input line does not split it for you. You get to split it
61 to an array yourself. And the split() operator has different
62 arguments than B<awk>'s.
66 The current input line is normally in $_, not $0. It generally does
67 not have the newline stripped. ($0 is the name of the program
68 executed.) See L<perlvar>.
72 $E<lt>I<digit>E<gt> does not refer to fields--it refers to substrings matched
73 by the last match pattern.
77 The print() statement does not add field and record separators unless
78 you set C<$,> and C<$\>. You can set $OFS and $ORS if you're using
83 You must open your files before you print to them.
87 The range operator is "..", not comma. The comma operator works as in
92 The match operator is "=~", not "~". ("~" is the one's complement
97 The exponentiation operator is "**", not "^". "^" is the XOR
98 operator, as in C. (You know, one could get the feeling that B<awk> is
99 basically incompatible with C.)
103 The concatenation operator is ".", not the null string. (Using the
104 null string would render C</pat/ /pat/> unparsable, because the third slash
105 would be interpreted as a division operator--the tokenizer is in fact
106 slightly context sensitive for operators like "/", "?", and "E<gt>".
107 And in fact, "." itself can be the beginning of a number.)
111 The C<next>, C<exit>, and C<continue> keywords work differently.
116 The following variables work differently:
119 ARGC $#ARGV or scalar @ARGV
123 FS (whatever you like)
124 NF $#Fld, or some such
136 You cannot set $RS to a pattern, only a string.
140 When in doubt, run the B<awk> construct through B<a2p> and see what it
147 Cerebral C programmers should take note of the following:
153 Curly brackets are required on C<if>'s and C<while>'s.
157 You must use C<elsif> rather than C<else if>.
161 The C<break> and C<continue> keywords from C become in
162 Perl C<last> and C<next>, respectively.
163 Unlike in C, these do I<not> work within a C<do { } while> construct.
167 There's no switch statement. (But it's easy to build one on the fly.)
171 Variables begin with "$", "@" or "%" in Perl.
175 C<printf()> does not implement the "*" format for interpolating
176 field widths, but it's trivial to use interpolation of double-quoted
177 strings to achieve the same effect.
181 Comments begin with "#", not "/*".
185 You can't take the address of anything, although a similar operator
186 in Perl is the backslash, which creates a reference.
190 C<ARGV> must be capitalized. C<$ARGV[0]> is C's C<argv[1]>, and C<argv[0]>
195 System calls such as link(), unlink(), rename(), etc. return nonzero for
200 Signal handlers deal with signal names, not numbers. Use C<kill -l>
201 to find their names on your system.
207 Seasoned B<sed> programmers should take note of the following:
213 Backreferences in substitutions use "$" rather than "\".
217 The pattern matching metacharacters "(", ")", and "|" do not have backslashes
222 The range operator is C<...>, rather than comma.
228 Sharp shell programmers should take note of the following:
234 The backtick operator does variable interpolation without regard to
235 the presence of single quotes in the command.
239 The backtick operator does no translation of the return value, unlike B<csh>.
243 Shells (especially B<csh>) do several levels of substitution on each
244 command line. Perl does substitution in only certain constructs
245 such as double quotes, backticks, angle brackets, and search patterns.
249 Shells interpret scripts a little bit at a time. Perl compiles the
250 entire program before executing it (except for C<BEGIN> blocks, which
251 execute at compile time).
255 The arguments are available via @ARGV, not $1, $2, etc.
259 The environment is not automatically made available as separate scalar
266 Practicing Perl Programmers should take note of the following:
272 Remember that many operations behave differently in a list
273 context than they do in a scalar one. See L<perldata> for details.
277 Avoid barewords if you can, especially all lowercase ones.
278 You can't tell by just looking at it whether a bareword is
279 a function or a string. By using quotes on strings and
280 parentheses on function calls, you won't ever get them confused.
284 You cannot discern from mere inspection which builtins
285 are unary operators (like chop() and chdir())
286 and which are list operators (like print() and unlink()).
287 (User-defined subroutines can be B<only> list operators, never
288 unary ones.) See L<perlop>.
292 People have a hard time remembering that some functions
293 default to $_, or @ARGV, or whatever, but that others which
294 you might expect to do not.
298 The E<lt>FHE<gt> construct is not the name of the filehandle, it is a readline
299 operation on that handle. The data read is assigned to $_ only if the
300 file read is the sole condition in a while loop:
303 while (defined($_ = <FH>)) { }..
304 <FH>; # data discarded!
308 Remember not to use C<=> when you need C<=~>;
309 these two constructs are quite different:
316 The C<do {}> construct isn't a real loop that you can use
321 Use C<my()> for local variables whenever you can get away with
322 it (but see L<perlform> for where you can't).
323 Using C<local()> actually gives a local value to a global
324 variable, which leaves you open to unforeseen side-effects
329 If you localize an exported variable in a module, its exported value will
330 not change. The local name becomes an alias to a new value but the
331 external name is still an alias for the original.
335 =head2 Perl4 to Perl5 Traps
337 Practicing Perl4 Programmers should take note of the following
338 Perl4-to-Perl5 specific traps.
340 They're crudely ordered according to the following list:
344 =item Discontinuance, Deprecation, and BugFix traps
346 Anything that's been fixed as a perl4 bug, removed as a perl4 feature
347 or deprecated as a perl4 feature with the intent to encourage usage of
348 some other perl5 feature.
352 Traps that appear to stem from the new parser.
354 =item Numerical Traps
356 Traps having to do with numerical or mathematical operators.
358 =item General data type traps
360 Traps involving perl standard data types.
362 =item Context Traps - scalar, list contexts
364 Traps related to context within lists, scalar statements/declarations.
366 =item Precedence Traps
368 Traps related to the precedence of parsing, evaluation, and execution of
371 =item General Regular Expression Traps using s///, etc.
373 Traps related to the use of pattern matching.
375 =item Subroutine, Signal, Sorting Traps
377 Traps related to the use of signals and signal handlers, general subroutines,
378 and sorting, along with sorting subroutines.
386 Traps specific to the use of C<dbmopen()>, and specific dbm implementations.
388 =item Unclassified Traps
394 If you find an example of a conversion trap that is not listed here,
395 please submit it to Bill Middleton <F<wjm@best.com>> for inclusion.
396 Also note that at least some of these can be caught with B<-w>.
398 =head2 Discontinuance, Deprecation, and BugFix traps
400 Anything that has been discontinued, deprecated, or fixed as
405 =item * Discontinuance
407 Symbols starting with "_" are no longer forced into package main, except
408 for C<$_> itself (and C<@_>, etc.).
414 print "\$_legacy is ",$_legacy,"\n";
416 # perl4 prints: $_legacy is 1
417 # perl5 prints: $_legacy is
421 Double-colon is now a valid package separator in a variable name. Thus these
422 behave differently in perl4 vs. perl5, because the packages don't exist.
424 $a=1;$b=2;$c=3;$var=4;
426 print "$var::abc::xyz\n";
428 # perl4 prints: 1::2::3 4::abc::xyz
431 Given that C<::> is now the preferred package delimiter, it is debatable
432 whether this should be classed as a bug or not.
433 (The older package delimiter, ' ,is used here)
439 # perl5 prints: Can't find string terminator "'" anywhere before EOF
441 You can avoid this problem, and remain compatible with perl4, if you
442 always explicitly include the package name:
445 print "x=${main'x}\n" ;
447 Also see precedence traps, for parsing C<$:>.
451 The second and third arguments of C<splice()> are now evaluated in scalar
452 context (as the Camel says) rather than list context.
454 sub sub1{return(0,2) } # return a 2-element list
455 sub sub2{ return(1,2,3)} # return a 3-element list
456 @a1 = ("a","b","c","d","e");
457 @a2 = splice(@a1,&sub1,&sub2);
458 print join(' ',@a2),"\n";
461 # perl5 prints: c d e
463 =item * Discontinuance
465 You can't do a C<goto> into a block that is optimized away. Darn.
471 print "Here I is!\n";
474 # perl4 prints: Here I is!
475 # perl5 dumps core (SEGV)
477 =item * Discontinuance
479 It is no longer syntactically legal to use whitespace as the name
480 of a variable, or as a delimiter for any kind of quote construct.
485 print "a is $a, b is $b\n";
487 # perl4 prints: a is foo bar, b is baz
488 # perl5 errors: Bareword found where operator expected
490 =item * Discontinuance
492 The archaic while/if BLOCK BLOCK syntax is no longer supported.
501 # perl4 prints: True!
502 # perl5 errors: syntax error at test.pl line 1, near "if {"
506 The C<**> operator now binds more tightly than unary minus.
507 It was documented to work this way before, but didn't.
514 =item * Discontinuance
516 The meaning of C<foreach{}> has changed slightly when it is iterating over a
517 list which is not an array. This used to assign the list to a
518 temporary array, but no longer does so (for efficiency). This means
519 that you'll now be iterating over the actual values, not over copies of
520 the values. Modifications to the loop variable can change the original
523 @list = ('ab','abc','bcd','def');
524 foreach $var (grep(/ab/,@list)){
527 print (join(':',@list));
529 # perl4 prints: ab:abc:bcd:def
530 # perl5 prints: 1:1:bcd:def
532 To retain Perl4 semantics you need to assign your list
533 explicitly to a temporary array and then iterate over that. For
534 example, you might need to change
536 foreach $var (grep(/ab/,@list)){
540 foreach $var (@tmp = grep(/ab/,@list)){
542 Otherwise changing $var will clobber the values of @list. (This most often
543 happens when you use C<$_> for the loop variable, and call subroutines in
544 the loop that don't properly localize C<$_>.)
546 =item * Discontinuance
548 C<split> with no arguments now behaves like C<split ' '> (which doesn't
549 return an initial null field if $_ starts with whitespace), it used to
550 behave like C<split /\s+/> (which does).
553 print join(':', split);
555 # perl4 prints: :hi:mom
556 # perl5 prints: hi:mom
560 Perl 4 would ignore any text which was attached to an B<-e> switch,
561 always taking the code snippet from the following arg. Additionally, it
562 would silently accept an B<-e> switch without a following arg. Both of
563 these behaviors have been fixed.
565 perl -e'print "attached to -e"' 'print "separate arg"'
567 # perl4 prints: separate arg
568 # perl5 prints: attached to -e
573 # perl5 dies: No code specified for -e.
575 =item * Discontinuance
577 In Perl 4 the return value of C<push> was undocumented, but it was
578 actually the last value being pushed onto the target list. In Perl 5
579 the return value of C<push> is documented, but has changed, it is the
580 number of elements in the resulting list.
583 print push(@x, 'first new', 'second new');
585 # perl4 prints: second new
588 =item * Discontinuance
590 In Perl 4 (and versions of Perl 5 before 5.004), C<'\r'> characters in
591 Perl code were silently allowed, although they could cause (mysterious!)
592 failures in certain constructs, particularly here documents. Now,
593 C<'\r'> characters cause an immediate fatal error. (Note: In this
594 example, the notation B<\015> represents the incorrect line
595 ending. Depending upon your text viewer, it will look different.)
600 # perl4 prints: foobar
601 # perl5.003 prints: foobar
602 # perl5.004 dies: Illegal character \015 (carriage return)
604 See L<perldiag> for full details.
608 Some error messages will be different.
610 =item * Discontinuance
612 Some bugs may have been inadvertently removed. :-)
618 Perl4-to-Perl5 traps from having to do with parsing.
624 Note the space between . and =
626 $string . = "more string";
629 # perl4 prints: more string
630 # perl5 prints: syntax error at - line 1, near ". ="
634 Better parsing in perl 5
638 print("hello, world\n");
640 # perl4 prints: hello, world
641 # perl5 prints: syntax error
645 "if it looks like a function, it is a function" rule.
648 ($foo == 1) ? "is one\n" : "is zero\n";
650 # perl4 prints: is zero
651 # perl5 warns: "Useless use of a constant in void context" if using -w
655 String interpolation of the C<$#array> construct differs when braces
656 are to used around the name.
662 # perl5 fails with syntax error
672 =head2 Numerical Traps
674 Perl4-to-Perl5 traps having to do with numerical operators,
675 operands, or output from same.
681 Formatted output and significant digits
683 print 7.373504 - 0, "\n";
684 printf "%20.18f\n", 7.373504 - 0;
696 This specific item has been deleted. It demonstrated how the auto-increment
697 operator would not catch when a number went over the signed int limit. Fixed
698 in version 5.003_04. But always be wary when using large integers.
705 Assignment of return values from numeric equality tests
706 does not work in perl5 when the test evaluates to false (0).
707 Logical tests now return an null, instead of 0
715 Also see L<"General Regular Expression Traps using s///, etc.">
716 for another example of this new feature...
718 =item * Bitwise string ops
720 When bitwise operators which can operate upon either numbers or
721 strings (C<& | ^ ~>) are given only strings as arguments, perl4 would
722 treat the operands as bitstrings so long as the program contained a call
723 to the C<vec()> function. perl5 treats the string operands as bitstrings.
724 (See L<perlop/Bitwise String Operators> for more details.)
728 $betty = $fred & $barney;
730 # Uncomment the next line to change perl4's behavior
731 # ($dummy) = vec("dummy", 0, 0);
739 # If vec() is used anywhere in the program, both print:
744 =head2 General data type traps
746 Perl4-to-Perl5 traps involving most data-types, and their usage
747 within certain expressions and/or context.
753 Negative array subscripts now count from the end of the array.
755 @a = (1, 2, 3, 4, 5);
756 print "The third element of the array is $a[3] also expressed as $a[-2] \n";
758 # perl4 prints: The third element of the array is 4 also expressed as
759 # perl5 prints: The third element of the array is 4 also expressed as 4
763 Setting C<$#array> lower now discards array elements, and makes them
764 impossible to recover.
767 print "Before: ",join('',@a);
769 print ", After: ",join('',@a);
771 print ", Recovered: ",join('',@a),"\n";
773 # perl4 prints: Before: abcde, After: ab, Recovered: abcd
774 # perl5 prints: Before: abcde, After: ab, Recovered: ab
778 Hashes get defined before use
781 die "scalar \$s defined" if defined($s);
782 die "array \@a defined" if defined(@a);
783 die "hash \%h defined" if defined(%h);
786 # perl5 dies: hash %h defined
788 Perl will now generate a warning when it sees defined(@a) and
793 glob assignment from variable to variable will fail if the assigned
794 variable is localized subsequent to the assignment
796 @a = ("This is Perl 4");
801 # perl4 prints: This is Perl 4
806 Assigning C<undef> to a glob has no effect in Perl 5. In Perl 4
807 it undefines the associated scalar (but may have other side effects
810 =item * (Scalar String)
812 Changes in unary negation (of strings)
813 This change effects both the return value and what it
814 does to auto(magic)increment.
821 # perl4 prints: aab : -0 : 1
822 # perl5 prints: aab : -aab : aac
826 perl 4 lets you modify constants:
830 for ($x = 0; $x < 3; $x++) {
834 print "before: $_[0]";
836 print " after: $_[0]\n";
847 # Modification of a read-only value attempted at foo.pl line 12.
852 The behavior is slightly different for:
854 print "$x", defined $x
857 # perl 5: <no output, $x is not called into existence>
859 =item * (Variable Suicide)
861 Variable suicide behavior is more consistent under Perl 5.
862 Perl5 exhibits the same behavior for hashes and scalars,
863 that perl4 exhibits for only scalars.
865 $aGlobal{ "aKey" } = "global value";
866 print "MAIN:", $aGlobal{"aKey"}, "\n";
871 local( *theArgument ) = @_;
872 local( %aNewLocal ); # perl 4 != 5.001l,m
873 $aNewLocal{"aKey"} = "this should never appear";
874 print "SUB: ", $theArgument{"aKey"}, "\n";
875 $aNewLocal{"aKey"} = "level $GlobalLevel"; # what should print
877 if( $GlobalLevel<4 ) {
892 # SUB: this should never appear
893 # SUB: this should never appear
894 # SUB: this should never appear
898 =head2 Context Traps - scalar, list contexts
902 =item * (list context)
904 The elements of argument lists for formats are now evaluated in list
905 context. This means you can interpolate list values now.
907 @fmt = ("foo","bar","baz");
914 # perl4 errors: Please use commas to separate fields in file
915 # perl5 prints: foo bar baz
917 =item * (scalar context)
919 The C<caller()> function now returns a false value in a scalar context
920 if there is no caller. This lets library files determine if they're
923 caller() ? (print "You rang?\n") : (print "Got a 0\n");
925 # perl4 errors: There is no caller
926 # perl5 prints: Got a 0
928 =item * (scalar context)
930 The comma operator in a scalar context is now guaranteed to give a
931 scalar context to its arguments.
937 # Perl4 prints: x = c # Thinks list context interpolates list
938 # Perl5 prints: x = 3 # Knows scalar uses length of list
940 =item * (list, builtin)
942 C<sprintf()> funkiness (array argument converted to scalar array count)
943 This test could be added to t/op/sprintf.t
945 @z = ('%s%s', 'foo', 'bar');
947 if ($x eq 'foobar') {print "ok 2\n";} else {print "not ok 2 '$x'\n";}
950 # perl5 prints: not ok 2
952 C<printf()> works fine, though:
957 # perl4 prints: foobar
958 # perl5 prints: foobar
964 =head2 Precedence Traps
966 Perl4-to-Perl5 traps involving precedence order.
968 Perl 4 has almost the same precedence rules as Perl 5 for the operators
969 that they both have. Perl 4 however, seems to have had some
970 inconsistencies that made the behavior differ from what was documented.
976 LHS vs. RHS of any assignment operator. LHS is evaluated first
977 in perl4, second in perl5; this can affect the relationship
978 between side-effects in sub-expressions.
980 @arr = ( 'left', 'right' );
981 $a{shift @arr} = shift @arr;
982 print join( ' ', keys %a );
985 # perl5 prints: right
989 These are now semantic errors because of precedence:
992 %map = ("a",1,"b",2,"c",3,"d",4);
993 $n = shift @list + 2; # first item in list plus 2
995 $m = keys %map + 2; # number of items in hash plus 2
998 # perl4 prints: n is 3, m is 6
999 # perl5 errors and fails to compile
1003 The precedence of assignment operators is now the same as the precedence
1004 of assignment. Perl 4 mistakenly gave them the precedence of the associated
1005 operator. So you now must parenthesize them in expressions like
1007 /foo/ ? ($a += 2) : ($a -= 2);
1011 /foo/ ? $a += 2 : $a -= 2
1013 would be erroneously parsed as
1015 (/foo/ ? $a += 2 : $a) -= 2;
1019 $a += /foo/ ? 1 : 2;
1021 now works as a C programmer would expect.
1027 is now incorrect. You need parentheses around the filehandle.
1028 Otherwise, perl5 leaves the statement as its default precedence:
1032 # perl4 opens or dies
1033 # perl5 errors: Precedence problem: open FOO should be open(FOO)
1037 perl4 gives the special variable, C<$:> precedence, where perl5
1038 treats C<$::> as main C<package>
1040 $a = "x"; print "$::a";
1042 # perl 4 prints: -:a
1047 perl4 had buggy precedence for the file test operators vis-a-vis
1048 the assignment operators. Thus, although the precedence table
1049 for perl4 leads one to believe C<-e $foo .= "q"> should parse as
1050 C<((-e $foo) .= "q")>, it actually parses as C<(-e ($foo .= "q"))>.
1051 In perl5, the precedence is as documented.
1055 # perl4 prints: no output
1056 # perl5 prints: Can't modify -e in concatenation
1060 In perl4, keys(), each() and values() were special high-precedence operators
1061 that operated on a single hash, but in perl5, they are regular named unary
1062 operators. As documented, named unary operators have lower precedence
1063 than the arithmetic and concatenation operators C<+ - .>, but the perl4
1064 variants of these operators actually bind tighter than C<+ - .>.
1071 # perl5 prints: Type of arg 1 to keys must be hash (not subtraction)
1073 The perl4 behavior was probably more useful, if less consistent.
1077 =head2 General Regular Expression Traps using s///, etc.
1079 All types of RE traps.
1083 =item * Regular Expression
1085 C<s'$lhs'$rhs'> now does no interpolation on either side. It used to
1086 interpolate $lhs but not $rhs. (And still does not match a literal
1090 $string = '1 2 $a $b';
1091 $string =~ s'$a'$b';
1094 # perl4 prints: $b 2 $a $b
1095 # perl5 prints: 1 2 $a $b
1097 =item * Regular Expression
1099 C<m//g> now attaches its state to the searched string rather than the
1100 regular expression. (Once the scope of a block is left for the sub, the
1101 state of the searched string is lost)
1107 sub doit{local($_) = shift; print "Got $_ "}
1109 # perl4 prints: blah blah blah
1110 # perl5 prints: infinite loop blah...
1112 =item * Regular Expression
1114 Currently, if you use the C<m//o> qualifier on a regular expression
1115 within an anonymous sub, I<all> closures generated from that anonymous
1116 sub will use the regular expression as it was compiled when it was used
1117 the very first time in any such closure. For instance, if you say
1120 my($left,$right) = @_;
1121 return sub { $_[0] =~ /$left stuff $right/o; };
1124 build_match() will always return a sub which matches the contents of
1125 $left and $right as they were the I<first> time that build_match()
1126 was called, not as they are in the current call.
1128 This is probably a bug, and may change in future versions of Perl.
1130 =item * Regular Expression
1132 If no parentheses are used in a match, Perl4 sets C<$+> to
1133 the whole match, just like C<$&>. Perl5 does not.
1138 # perl4 prints: bcde
1141 =item * Regular Expression
1143 substitution now returns the null string if it fails
1146 $value = ($string =~ s/foo//);
1152 Also see L<Numerical Traps> for another example of this new feature.
1154 =item * Regular Expression
1156 C<s`lhs`rhs`> (using backticks) is now a normal substitution, with no
1160 $string =~ s`^`hostname`;
1161 print $string, "\n";
1163 # perl4 prints: <the local hostname>
1164 # perl5 prints: hostname
1166 =item * Regular Expression
1168 Stricter parsing of variables used in regular expressions
1170 s/^([^$grpc]*$grpc[$opt$plus$rep]?)//o;
1172 # perl4: compiles w/o error
1173 # perl5: with Scalar found where operator expected ..., near "$opt$plus"
1175 an added component of this example, apparently from the same script, is
1176 the actual value of the s'd string after the substitution.
1177 C<[$opt]> is a character class in perl4 and an array subscript in perl5
1182 s/^([^$grpc]*$grpc[$opt]?)/foo/;
1186 # perl5 prints: foobar
1188 =item * Regular Expression
1190 Under perl5, C<m?x?> matches only once, like C<?x?>. Under perl4, it matched
1191 repeatedly, like C</x/> or C<m!x!>.
1194 sub match { $test =~ m?once?; }
1197 # m?x? matches more then once
1200 # m?x? matches only once
1204 # perl4 prints: perl4
1205 # perl5 prints: perl5
1210 =head2 Subroutine, Signal, Sorting Traps
1212 The general group of Perl4-to-Perl5 traps having to do with
1213 Signals, Sorting, and their related subroutines, as well as
1214 general subroutine traps. Includes some OS-Specific traps.
1220 Barewords that used to look like strings to Perl will now look like subroutine
1221 calls if a subroutine by that name is defined before the compiler sees them.
1223 sub SeeYa { warn"Hasta la vista, baby!" }
1224 $SIG{'TERM'} = SeeYa;
1225 print "SIGTERM is now $SIG{'TERM'}\n";
1227 # perl4 prints: SIGTERM is main'SeeYa
1228 # perl5 prints: SIGTERM is now main::1
1230 Use B<-w> to catch this one
1232 =item * (Sort Subroutine)
1234 reverse is no longer allowed as the name of a sort subroutine.
1236 sub reverse{ print "yup "; $a <=> $b }
1237 print sort reverse a,b,c;
1239 # perl4 prints: yup yup yup yup abc
1242 =item * warn() won't let you specify a filehandle.
1244 Although it _always_ printed to STDERR, warn() would let you specify a
1245 filehandle in perl4. With perl5 it does not.
1249 # perl4 prints: Foo!
1250 # perl5 prints: String found where operator expected
1260 Under HPUX, and some other SysV OSes, one had to reset any signal handler,
1261 within the signal handler function, each time a signal was handled with
1262 perl4. With perl5, the reset is now done correctly. Any code relying
1263 on the handler _not_ being reset will have to be reworked.
1265 Since version 5.002, Perl uses sigaction() under SysV.
1270 $SIG{'INT'} = 'gotit';
1279 while (1) {sleep(10);}
1282 # perl4 (HPUX) prints: Got INT...
1283 # perl5 (HPUX) prints: Got INT... Got INT...
1287 Under SysV OSes, C<seek()> on a file opened to append C<E<gt>E<gt>> now does
1288 the right thing w.r.t. the fopen() manpage. e.g., - When a file is opened
1289 for append, it is impossible to overwrite information already in
1292 open(TEST,">>seek.test");
1293 $start = tell TEST ;
1298 seek(TEST,$start,0);
1299 print TEST "18 characters here";
1301 # perl4 (solaris) seek.test has: 18 characters here
1302 # perl5 (solaris) seek.test has: 1 2 3 4 5 6 7 8 9 18 characters here
1308 =head2 Interpolation Traps
1310 Perl4-to-Perl5 traps having to do with how things get interpolated
1311 within certain expressions, statements, contexts, or whatever.
1315 =item * Interpolation
1317 @ now always interpolates an array in double-quotish strings.
1319 print "To: someone@somewhere.com\n";
1321 # perl4 prints: To:someone@somewhere.com
1322 # perl5 errors : In string, @somewhere now must be written as \@somewhere
1324 =item * Interpolation
1326 Double-quoted strings may no longer end with an unescaped $ or @.
1330 print "foo is $foo, bar is $bar\n";
1332 # perl4 prints: foo is foo$, bar is bar@
1333 # perl5 errors: Final $ should be \$ or $name
1335 Note: perl5 DOES NOT error on the terminating @ in $bar
1337 =item * Interpolation
1339 Perl now sometimes evaluates arbitrary expressions inside braces that occur
1340 within double quotes (usually when the opening brace is preceded by C<$>
1346 sub foo { return "bar" };
1347 print "|@{w.w.w}|${main'foo}|";
1349 # perl4 prints: |@{w.w.w}|foo|
1350 # perl5 prints: |buz|bar|
1352 Note that you can C<use strict;> to ward off such trappiness under perl5.
1354 =item * Interpolation
1356 The construct "this is $$x" used to interpolate the pid at that
1357 point, but now apparently tries to dereference $x. C<$$> by itself still
1358 works fine, however.
1360 print "this is $$x\n";
1362 # perl4 prints: this is XXXx (XXX is the current pid)
1363 # perl5 prints: this is
1365 =item * Interpolation
1367 Creation of hashes on the fly with C<eval "EXPR"> now requires either both
1368 C<$>'s to be protected in the specification of the hash name, or both curlies
1369 to be protected. If both curlies are protected, the result will be compatible
1370 with perl4 and perl5. This is a very common practice, and should be changed
1371 to use the block form of C<eval{}> if possible.
1373 $hashname = "foobar";
1376 eval "\$$hashname{'$key'} = q|$value|";
1377 (defined($foobar{'baz'})) ? (print "Yup") : (print "Nope");
1380 # perl5 prints: Nope
1384 eval "\$$hashname{'$key'} = q|$value|";
1388 eval "\$\$hashname{'$key'} = q|$value|";
1390 causes the following result:
1392 # perl4 prints: Nope
1397 eval "\$$hashname\{'$key'\} = q|$value|";
1399 causes the following result:
1403 # and is compatible for both versions
1406 =item * Interpolation
1408 perl4 programs which unconsciously rely on the bugs in earlier perl versions.
1410 perl -e '$bar=q/not/; print "This is $foo{$bar} perl5"'
1412 # perl4 prints: This is not perl5
1413 # perl5 prints: This is perl5
1415 =item * Interpolation
1417 You also have to be careful about array references.
1422 perl 5 prints: syntax error
1424 =item * Interpolation
1426 Similarly, watch out for:
1429 print "\$$foo{bar}\n";
1431 # perl4 prints: $array{bar}
1434 Perl 5 is looking for C<$array{bar}> which doesn't exist, but perl 4 is
1435 happy just to expand $foo to "array" by itself. Watch out for this
1436 especially in C<eval>'s.
1438 =item * Interpolation
1440 C<qq()> string passed to C<eval>
1443 foreach \$y (keys %\$x\) {
1448 # perl4 runs this ok
1449 # perl5 prints: Can't find string terminator ")"
1461 Existing dbm databases created under perl4 (or any other dbm/ndbm tool)
1462 may cause the same script, run under perl5, to fail. The build of perl5
1463 must have been linked with the same dbm/ndbm as the default for C<dbmopen()>
1464 to function properly without C<tie>'ing to an extension dbm implementation.
1466 dbmopen (%dbm, "file", undef);
1470 # perl5 prints: ok (IFF linked with -ldbm or -lndbm)
1475 Existing dbm databases created under perl4 (or any other dbm/ndbm tool)
1476 may cause the same script, run under perl5, to fail. The error generated
1477 when exceeding the limit on the key/value size will cause perl5 to exit
1480 dbmopen(DB, "testdb",0600) || die "couldn't open db! $!";
1481 $DB{'trap'} = "x" x 1024; # value too large for most dbm/ndbm
1485 dbm store returned -1, errno 28, key "trap" at - line 3.
1489 dbm store returned -1, errno 28, key "trap" at - line 3.
1493 =head2 Unclassified Traps
1499 =item * C<require>/C<do> trap using returned value
1501 If the file doit.pl has:
1509 And the do.pl file has the following single line:
1513 Running doit.pl gives the following:
1515 # perl 4 prints: 3 (aborts the subroutine early)
1518 Same behavior if you replace C<do> with C<require>.
1520 =item * C<split> on empty string with LIMIT specified
1523 @list = split(/foo/, $string, 2)
1525 Perl4 returns a one element list containing the empty string but Perl5
1526 returns an empty list.
1530 As always, if any of these are ever officially declared as bugs,
1531 they'll be fixed and removed.