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>.
13 Accustomed B<awk> users should take special note of the following:
19 The English module, loaded via
23 allows you to refer to special variables (like $RS) as
24 though they were in B<awk>; see L<perlvar> for details.
28 Semicolons are required after all simple statements in Perl (except
29 at the end of a block). Newline is not a statement delimiter.
33 Curly brackets are required on C<if>s and C<while>s.
37 Variables begin with "$" or "@" in Perl.
41 Arrays index from 0. Likewise string positions in substr() and
46 You have to decide whether your array has numeric or string indices.
50 Associative array values do not spring into existence upon mere
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 yourself to an array. And the split() operator has different
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, since the third slash
105 would be interpreted as a division operator--the tokener 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 5 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 only in 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 lower-case ones.
278 You can't tell just by looking at it whether a bareword is
279 a function or a string. By using quotes on strings and
280 parens on function calls, you won't ever get them confused.
284 You cannot discern from mere inspection which built-ins
285 are unary operators (like chop() and chdir())
286 and which are list operators (like print() and unlink()).
287 (User-defined subroutines can B<only> be 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 only assigned to $_ if the
300 file read is the sole condition in a while loop:
303 while ($_ = <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 C<-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, since the packages don't exist.
424 $a=1;$b=2;$c=3;$var=4;
426 print "$var::abc::xyz\n";
428 # perl4 prints: 1::2::3 4::abc::xyz
431 Given that C<::> is now the preferred package delimiter, it is debatable
432 whether this should be classed as a bug or not.
433 (The older package delimiter, ' ,is used here)
439 # perl5 prints: Can't find string terminator "'" anywhere before EOF
441 Also see precedence traps, for parsing C<$:>.
445 The second and third arguments of C<splice()> are now evaluated in scalar
446 context (as the Camel says) rather than list context.
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";
455 # perl5 prints: c d e
457 =item * Discontinuance
459 You can't do a C<goto> into a block that is optimized away. Darn.
465 print "Here I is!\n";
468 # perl4 prints: Here I is!
469 # perl5 dumps core (SEGV)
471 =item * Discontinuance
473 It is no longer syntactically legal to use whitespace as the name
474 of a variable, or as a delimiter for any kind of quote construct.
479 print "a is $a, b is $b\n";
481 # perl4 prints: a is foo bar, b is baz
482 # perl5 errors: Bare word found where operator expected
484 =item * Discontinuance
486 The archaic while/if BLOCK BLOCK syntax is no longer supported.
495 # perl4 prints: True!
496 # perl5 errors: syntax error at test.pl line 1, near "if {"
500 The C<**> operator now binds more tightly than unary minus.
501 It was documented to work this way before, but didn't.
508 =item * Discontinuance
510 The meaning of C<foreach{}> has changed slightly when it is iterating over a
511 list which is not an array. This used to assign the list to a
512 temporary array, but no longer does so (for efficiency). This means
513 that you'll now be iterating over the actual values, not over copies of
514 the values. Modifications to the loop variable can change the original
517 @list = ('ab','abc','bcd','def');
518 foreach $var (grep(/ab/,@list)){
521 print (join(':',@list));
523 # perl4 prints: ab:abc:bcd:def
524 # perl5 prints: 1:1:bcd:def
526 To retain Perl4 semantics you need to assign your list
527 explicitly to a temporary array and then iterate over that. For
528 example, you might need to change
530 foreach $var (grep(/ab/,@list)){
534 foreach $var (@tmp = grep(/ab/,@list)){
536 Otherwise changing $var will clobber the values of @list. (This most often
537 happens when you use C<$_> for the loop variable, and call subroutines in
538 the loop that don't properly localize C<$_>.)
540 =item * Discontinuance
542 C<split> with no arguments now behaves like C<split ' '> (which doesn't
543 return an initial null field if $_ starts with whitespace), it used to
544 behave like C<split /\s+/> (which does).
547 print join(':', split);
549 # perl4 prints: :hi:mom
550 # perl5 prints: hi:mom
554 Some error messages will be different.
556 =item * Discontinuance
558 Some bugs may have been inadvertently removed. :-)
564 Perl4-to-Perl5 traps from having to do with parsing.
570 Note the space between . and =
572 $string . = "more string";
575 # perl4 prints: more string
576 # perl5 prints: syntax error at - line 1, near ". ="
580 Better parsing in perl 5
584 print("hello, world\n");
586 # perl4 prints: hello, world
587 # perl5 prints: syntax error
591 "if it looks like a function, it is a function" rule.
594 ($foo == 1) ? "is one\n" : "is zero\n";
596 # perl4 prints: is zero
597 # perl5 warns: "Useless use of a constant in void context" if using -w
601 =head2 Numerical Traps
603 Perl4-to-Perl5 traps having to do with numerical operators,
604 operands, or output from same.
610 Formatted output and significant digits
612 print 7.373504 - 0, "\n";
613 printf "%20.18f\n", 7.373504 - 0;
625 This specific item has been deleted. It demonstrated how the autoincrement
626 operator would not catch when a number went over the signed int limit. Fixed
627 in 5.003_04. But always be wary when using large ints. If in doubt:
633 Assignment of return values from numeric equality tests
634 does not work in perl5 when the test evaluates to false (0).
635 Logical tests now return an null, instead of 0
643 Also see the L<General Regular Expression Traps> tests for another example
644 of this new feature...
648 =head2 General data type traps
650 Perl4-to-Perl5 traps involving most data-types, and their usage
651 within certain expressions and/or context.
657 Negative array subscripts now count from the end of the array.
659 @a = (1, 2, 3, 4, 5);
660 print "The third element of the array is $a[3] also expressed as $a[-2] \n";
662 # perl4 prints: The third element of the array is 4 also expressed as
663 # perl5 prints: The third element of the array is 4 also expressed as 4
667 Setting C<$#array> lower now discards array elements, and makes them
668 impossible to recover.
671 print "Before: ",join('',@a);
673 print ", After: ",join('',@a);
675 print ", Recovered: ",join('',@a),"\n";
677 # perl4 prints: Before: abcde, After: ab, Recovered: abcd
678 # perl5 prints: Before: abcde, After: ab, Recovered: ab
682 Hashes get defined before use
685 die "scalar \$s defined" if defined($s);
686 die "array \@a defined" if defined(@a);
687 die "hash \%h defined" if defined(%h);
690 # perl5 dies: hash %h defined
694 glob assignment from variable to variable will fail if the assigned
695 variable is localized subsequent to the assignment
697 @a = ("This is Perl 4");
702 # perl4 prints: This is Perl 4
707 *fred = *barney; # fred is aliased to barney
710 print "@fred"; # should print "1, 2, 4"
712 # perl4 prints: 1 2 4
713 # perl5 prints: Literal @fred now requires backslash
715 =item * (Scalar String)
717 Changes in unary negation (of strings)
718 This change effects both the return value and what it
719 does to auto(magic)increment.
726 # perl4 prints: aab : -0 : 1
727 # perl5 prints: aab : -aab : aac
731 perl 4 lets you modify constants:
735 for ($x = 0; $x < 3; $x++) {
739 print "before: $_[0]";
741 print " after: $_[0]\n";
752 # Modification of a read-only value attempted at foo.pl line 12.
757 The behavior is slightly different for:
759 print "$x", defined $x
762 # perl 5: <no output, $x is not called into existence>
764 =item * (Variable Suicide)
766 Variable suicide behavior is more consistent under Perl 5.
767 Perl5 exhibits the same behavior for associative arrays and scalars,
768 that perl4 exhibits only for scalars.
770 $aGlobal{ "aKey" } = "global value";
771 print "MAIN:", $aGlobal{"aKey"}, "\n";
776 local( *theArgument ) = @_;
777 local( %aNewLocal ); # perl 4 != 5.001l,m
778 $aNewLocal{"aKey"} = "this should never appear";
779 print "SUB: ", $theArgument{"aKey"}, "\n";
780 $aNewLocal{"aKey"} = "level $GlobalLevel"; # what should print
782 if( $GlobalLevel<4 ) {
797 # SUB: this should never appear
798 # SUB: this should never appear
799 # SUB: this should never appear
803 =head2 Context Traps - scalar, list contexts
807 =item * (list context)
809 The elements of argument lists for formats are now evaluated in list
810 context. This means you can interpolate list values now.
812 @fmt = ("foo","bar","baz");
819 # perl4 errors: Please use commas to separate fields in file
820 # perl5 prints: foo bar baz
822 =item * (scalar context)
824 The C<caller()> function now returns a false value in a scalar context
825 if there is no caller. This lets library files determine if they're
828 caller() ? (print "You rang?\n") : (print "Got a 0\n");
830 # perl4 errors: There is no caller
831 # perl5 prints: Got a 0
833 =item * (scalar context)
835 The comma operator in a scalar context is now guaranteed to give a
836 scalar context to its arguments.
842 # Perl4 prints: x = c # Thinks list context interpolates list
843 # Perl5 prints: x = 3 # Knows scalar uses length of list
845 =item * (list, builtin)
847 C<sprintf()> funkiness (array argument converted to scalar array count)
848 This test could be added to t/op/sprintf.t
850 @z = ('%s%s', 'foo', 'bar');
852 if ($x eq 'foobar') {print "ok 2\n";} else {print "not ok 2 '$x'\n";}
855 # perl5 prints: not ok 2
857 C<printf()> works fine, though:
862 # perl4 prints: foobar
863 # perl5 prints: foobar
869 =head2 Precedence Traps
871 Perl4-to-Perl5 traps involving precedence order.
877 LHS vs. RHS when both sides are getting an op.
879 @arr = ( 'left', 'right' );
880 $a{shift @arr} = shift @arr;
881 print join( ' ', keys %a );
884 # perl5 prints: right
888 These are now semantic errors because of precedence:
891 %map = ("a",1,"b",2,"c",3,"d",4);
892 $n = shift @list + 2; # first item in list plus 2
894 $m = keys %map + 2; # number of items in hash plus 2
897 # perl4 prints: n is 3, m is 6
898 # perl5 errors and fails to compile
902 The precedence of assignment operators is now the same as the precedence
903 of assignment. Perl 4 mistakenly gave them the precedence of the associated
904 operator. So you now must parenthesize them in expressions like
906 /foo/ ? ($a += 2) : ($a -= 2);
910 /foo/ ? $a += 2 : $a -= 2
912 would be erroneously parsed as
914 (/foo/ ? $a += 2 : $a) -= 2;
920 now works as a C programmer would expect.
926 is now incorrect. You need parens around the filehandle.
927 Otherwise, perl5 leaves the statement as it's default precedence:
931 # perl4 opens or dies
932 # perl5 errors: Precedence problem: open FOO should be open(FOO)
936 perl4 gives the special variable, C<$:> precedence, where perl5
937 treats C<$::> as main C<package>
939 $a = "x"; print "$::a";
946 concatenation precedence over filetest operator?
950 # perl4 prints: no output
951 # perl5 prints: Can't modify -e in concatenation
955 Assignment to value takes precedence over assignment to key in
956 perl5 when using the shift operator on both sides.
958 @arr = ( 'left', 'right' );
959 $a{shift @arr} = shift @arr;
960 print join( ' ', keys %a );
963 # perl5 prints: right
967 =head2 General Regular Expression Traps using s///, etc.
969 All types of RE traps.
973 =item * Regular Expression
975 C<s'$lhs'$rhs'> now does no interpolation on either side. It used to
976 interpolate C<$lhs> but not C<$rhs>. (And still does not match a literal
980 $string = '1 2 $a $b';
984 # perl4 prints: $b 2 $a $b
985 # perl5 prints: 1 2 $a $b
987 =item * Regular Expression
989 C<m//g> now attaches its state to the searched string rather than the
990 regular expression. (Once the scope of a block is left for the sub, the
991 state of the searched string is lost)
997 sub doit{local($_) = shift; print "Got $_ "}
999 # perl4 prints: blah blah blah
1000 # perl5 prints: infinite loop blah...
1002 =item * Regular Expression
1004 If no parentheses are used in a match, Perl4 sets C<$+> to
1005 the whole match, just like C<$&>. Perl5 does not.
1010 # perl4 prints: bcde
1013 =item * Regular Expression
1015 substitution now returns the null string if it fails
1018 $value = ($string =~ s/foo//);
1024 Also see L<Numerical Traps> for another example of this new feature.
1026 =item * Regular Expression
1028 C<s`lhs`rhs`> (using backticks) is now a normal substitution, with no
1032 $string =~ s`^`hostname`;
1033 print $string, "\n";
1035 # perl4 prints: <the local hostname>
1036 # perl5 prints: hostname
1038 =item * Regular Expression
1040 Stricter parsing of variables used in regular expressions
1042 s/^([^$grpc]*$grpc[$opt$plus$rep]?)//o;
1044 # perl4: compiles w/o error
1045 # perl5: with Scalar found where operator expected ..., near "$opt$plus"
1047 an added component of this example, apparently from the same script, is
1048 the actual value of the s'd string after the substitution.
1049 C<[$opt]> is a character class in perl4 and an array subscript in perl5
1054 s/^([^$grpc]*$grpc[$opt]?)/foo/;
1058 # perl5 prints: foobar
1060 =item * Regular Expression
1062 Under perl5, C<m?x?> matches only once, like C<?x?>. Under perl4, it matched
1063 repeatedly, like C</x/> or C<m!x!>.
1066 sub match { $test =~ m?once?; }
1069 # m?x? matches more then once
1072 # m?x? matches only once
1076 # perl4 prints: perl4
1077 # perl5 prints: perl5
1082 =head2 Subroutine, Signal, Sorting Traps
1084 The general group of Perl4-to-Perl5 traps having to do with
1085 Signals, Sorting, and their related subroutines, as well as
1086 general subroutine traps. Includes some OS-Specific traps.
1092 Barewords that used to look like strings to Perl will now look like subroutine
1093 calls if a subroutine by that name is defined before the compiler sees them.
1095 sub SeeYa { warn"Hasta la vista, baby!" }
1096 $SIG{'TERM'} = SeeYa;
1097 print "SIGTERM is now $SIG{'TERM'}\n";
1099 # perl4 prints: SIGTERM is main'SeeYa
1100 # perl5 prints: SIGTERM is now main::1
1102 Use B<-w> to catch this one
1104 =item * (Sort Subroutine)
1106 reverse is no longer allowed as the name of a sort subroutine.
1108 sub reverse{ print "yup "; $a <=> $b }
1109 print sort reverse a,b,c;
1111 # perl4 prints: yup yup yup yup abc
1114 =item * warn() won't let you specify a filehandle.
1116 Although it _always_ printed to STDERR, warn() would let you specify a
1117 filehandle in perl4. With perl5 it does not.
1121 # perl4 prints: Foo!
1122 # perl5 prints: String found where operator expected
1132 Under HPUX, and some other SysV OS's, one had to reset any signal handler,
1133 within the signal handler function, each time a signal was handled with
1134 perl4. With perl5, the reset is now done correctly. Any code relying
1135 on the handler _not_ being reset will have to be reworked.
1137 5.002 and beyond uses sigaction() under SysV
1142 $SIG{'INT'} = 'gotit';
1151 while (1) {sleep(10);}
1154 # perl4 (HPUX) prints: Got INT...
1155 # perl5 (HPUX) prints: Got INT... Got INT...
1159 Under SysV OS's, C<seek()> on a file opened to append C<E<gt>E<gt>> now does
1160 the right thing w.r.t. the fopen() man page. e.g. - When a file is opened
1161 for append, it is impossible to overwrite information already in
1164 open(TEST,">>seek.test");
1165 $start = tell TEST ;
1170 seek(TEST,$start,0);
1171 print TEST "18 characters here";
1173 # perl4 (solaris) seek.test has: 18 characters here
1174 # perl5 (solaris) seek.test has: 1 2 3 4 5 6 7 8 9 18 characters here
1180 =head2 Interpolation Traps
1182 Perl4-to-Perl5 traps having to do with how things get interpolated
1183 within certain expressions, statements, contexts, or whatever.
1187 =item * Interpolation
1189 @ now always interpolates an array in double-quotish strings.
1191 print "To: someone@somewhere.com\n";
1193 # perl4 prints: To:someone@somewhere.com
1194 # perl5 errors : Literal @somewhere now requires backslash
1196 =item * Interpolation
1198 Double-quoted strings may no longer end with an unescaped $ or @.
1202 print "foo is $foo, bar is $bar\n";
1204 # perl4 prints: foo is foo$, bar is bar@
1205 # perl5 errors: Final $ should be \$ or $name
1207 Note: perl5 DOES NOT error on the terminating @ in $bar
1209 =item * Interpolation
1211 Perl now sometimes evaluates arbitrary expressions inside braces that occur
1212 within double quotes (usually when the opening brace is preceded by C<$>
1218 sub foo { return "bar" };
1219 print "|@{w.w.w}|${main'foo}|";
1221 # perl4 prints: |@{w.w.w}|foo|
1222 # perl5 prints: |buz|bar|
1224 Note that you can C<use strict;> to ward off such trappiness under perl5.
1226 =item * Interpolation
1228 The construct "this is $$x" used to interpolate the pid at that
1229 point, but now apparently tries to dereference C<$x>. C<$$> by itself still
1230 works fine, however.
1232 print "this is $$x\n";
1234 # perl4 prints: this is XXXx (XXX is the current pid)
1235 # perl5 prints: this is
1237 =item * Interpolation
1239 Creation of hashes on the fly with C<eval "EXPR"> now requires either both
1240 C<$>'s to be protected in the specification of the hash name, or both curlies
1241 to be protected. If both curlies are protected, the result will be compatible
1242 with perl4 and perl5. This is a very common practice, and should be changed
1243 to use the block form of C<eval{}> if possible.
1245 $hashname = "foobar";
1248 eval "\$$hashname{'$key'} = q|$value|";
1249 (defined($foobar{'baz'})) ? (print "Yup") : (print "Nope");
1252 # perl5 prints: Nope
1256 eval "\$$hashname{'$key'} = q|$value|";
1260 eval "\$\$hashname{'$key'} = q|$value|";
1262 causes the following result:
1264 # perl4 prints: Nope
1269 eval "\$$hashname\{'$key'\} = q|$value|";
1271 causes the following result:
1275 # and is compatible for both versions
1278 =item * Interpolation
1280 perl4 programs which unconsciously rely on the bugs in earlier perl versions.
1282 perl -e '$bar=q/not/; print "This is $foo{$bar} perl5"'
1284 # perl4 prints: This is not perl5
1285 # perl5 prints: This is perl5
1287 =item * Interpolation
1289 You also have to be careful about array references.
1294 perl 5 prints: syntax error
1296 =item * Interpolation
1298 Similarly, watch out for:
1301 print "\$$foo{bar}\n";
1303 # perl4 prints: $array{bar}
1306 Perl 5 is looking for C<$array{bar}> which doesn't exist, but perl 4 is
1307 happy just to expand $foo to "array" by itself. Watch out for this
1308 especially in C<eval>'s.
1310 =item * Interpolation
1312 C<qq()> string passed to C<eval>
1315 foreach \$y (keys %\$x\) {
1320 # perl4 runs this ok
1321 # perl5 prints: Can't find string terminator ")"
1333 Existing dbm databases created under perl4 (or any other dbm/ndbm tool)
1334 may cause the same script, run under perl5, to fail. The build of perl5
1335 must have been linked with the same dbm/ndbm as the default for C<dbmopen()>
1336 to function properly without C<tie>'ing to an extension dbm implementation.
1338 dbmopen (%dbm, "file", undef);
1342 # perl5 prints: ok (IFF linked with -ldbm or -lndbm)
1347 Existing dbm databases created under perl4 (or any other dbm/ndbm tool)
1348 may cause the same script, run under perl5, to fail. The error generated
1349 when exceeding the limit on the key/value size will cause perl5 to exit
1352 dbmopen(DB, "testdb",0600) || die "couldn't open db! $!";
1353 $DB{'trap'} = "x" x 1024; # value too large for most dbm/ndbm
1357 dbm store returned -1, errno 28, key "trap" at - line 3.
1361 dbm store returned -1, errno 28, key "trap" at - line 3.
1365 =head2 Unclassified Traps
1371 =item * Unclassified
1373 C<require>/C<do> trap using returned value
1375 If the file doit.pl has:
1383 And the do.pl file has the following single line:
1387 Running doit.pl gives the following:
1389 # perl 4 prints: 3 (aborts the subroutine early)
1392 Same behavior if you replace C<do> with C<require>.
1396 As always, if any of these are ever officially declared as bugs,
1397 they'll be fixed and removed.