perltrap.pod

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    # perl4 opens or dies
    # perl5 opens FOO, dying only if 'FOO' is false, i.e. never

=item * Precedence

perl4 gives the special variable, C<$:> precedence, where perl5
treats C<$::> as main C<package>

    $a = "x"; print "$::a";

    # perl 4 prints: -:a
    # perl 5 prints: x

=item * Precedence

perl4 had buggy precedence for the file test operators vis-a-vis
the assignment operators.  Thus, although the precedence table
for perl4 leads one to believe C<-e $foo .= "q"> should parse as
C<((-e $foo) .= "q")>, it actually parses as C<(-e ($foo .= "q"))>.
In perl5, the precedence is as documented.

    -e $foo .= "q"

    # perl4 prints: no output
    # perl5 prints: Can't modify -e in concatenation

=item * Precedence

In perl4, keys(), each() and values() were special high-precedence operators
that operated on a single hash, but in perl5, they are regular named unary
operators.  As documented, named unary operators have lower precedence
than the arithmetic and concatenation operators C<+ - .>, but the perl4
variants of these operators actually bind tighter than C<+ - .>.
Thus, for:

    %foo = 1..10;
    print keys %foo - 1

    # perl4 prints: 4
    # perl5 prints: Type of arg 1 to keys must be hash (not subtraction)

The perl4 behavior was probably more useful, if less consistent.

=back

=head2 General Regular Expression Traps using s///, etc.

All types of RE traps.

=over 5

=item * Regular Expression

C<s'$lhs'$rhs'> now does no interpolation on either side.  It used to
interpolate $lhs but not $rhs.  (And still does not match a literal
'$' in string)

    $a=1;$b=2;
    $string = '1 2 $a $b';
    $string =~ s'$a'$b';
    print $string,"\n";

    # perl4 prints: $b 2 $a $b
    # perl5 prints: 1 2 $a $b

=item * Regular Expression

C<m//g> now attaches its state to the searched string rather than the
regular expression.  (Once the scope of a block is left for the sub, the
state of the searched string is lost)

    $_ = "ababab";
    while(m/ab/g){
        &doit("blah");
    }
    sub doit{local($_) = shift; print "Got $_ "}

    # perl4 prints: Got blah Got blah Got blah Got blah
    # perl5 prints: infinite loop blah...

=item * Regular Expression

Currently, if you use the C<m//o> qualifier on a regular expression
within an anonymous sub, I<all> closures generated from that anonymous
sub will use the regular expression as it was compiled when it was used
the very first time in any such closure.  For instance, if you say

    sub build_match {
        my($left,$right) = @_;
        return sub { $_[0] =~ /$left stuff $right/o; };
    }
    $good = build_match('foo','bar');
    $bad = build_match('baz','blarch');
    print $good->('foo stuff bar') ? "ok\n" : "not ok\n";
    print $bad->('baz stuff blarch') ? "ok\n" : "not ok\n";
    print $bad->('foo stuff bar') ? "not ok\n" : "ok\n";

For most builds of Perl5, this will print:
ok
not ok
not ok

build_match() will always return a sub which matches the contents of
$left and $right as they were the I<first> time that build_match()
was called, not as they are in the current call.

=item * Regular Expression

If no parentheses are used in a match, Perl4 sets C<$+> to
the whole match, just like C<$&>. Perl5 does not.

    "abcdef" =~ /b.*e/;
    print "\$+ = $+\n";

    # perl4 prints: bcde
    # perl5 prints:

=item * Regular Expression

substitution now returns the null string if it fails

    $string = "test";
    $value = ($string =~ s/foo//);
    print $value, "\n";

    # perl4 prints: 0
    # perl5 prints:

Also see L<Numerical Traps> for another example of this new feature.

=item * Regular Expression

C<s`lhs`rhs`> (using backticks) is now a normal substitution, with no
backtick expansion

    $string = "";
    $string =~ s`^`hostname`;
    print $string, "\n";

    # perl4 prints: <the local hostname>
    # perl5 prints: hostname

=item * Regular Expression

Stricter parsing of variables used in regular expressions

    s/^([^$grpc]*$grpc[$opt$plus$rep]?)//o;

    # perl4: compiles w/o error
    # perl5: with Scalar found where operator expected ..., near "$opt$plus"

an added component of this example, apparently from the same script, is
the actual value of the s'd string after the substitution.
C<[$opt]> is a character class in perl4 and an array subscript in perl5

    $grpc = 'a';
    $opt  = 'r';
    $_ = 'bar';
    s/^([^$grpc]*$grpc[$opt]?)/foo/;
    print ;

    # perl4 prints: foo
    # perl5 prints: foobar

=item * Regular Expression

Under perl5, C<m?x?> matches only once, like C<?x?>. Under perl4, it matched
repeatedly, like C</x/> or C<m!x!>.

    $test = "once";
    sub match { $test =~ m?once?; }
    &match();
    if( &match() ) {
        # m?x? matches more then once
        print "perl4\n";
    } else {
        # m?x? matches only once
        print "perl5\n";
    }

    # perl4 prints: perl4
    # perl5 prints: perl5


=back

=head2 Subroutine, Signal, Sorting Traps

The general group of Perl4-to-Perl5 traps having to do with
Signals, Sorting, and their related subroutines, as well as
general subroutine traps.  Includes some OS-Specific traps.

=over 5

=item * (Signals)

Barewords that used to look like strings to Perl will now look like subroutine
calls if a subroutine by that name is defined before the compiler sees them.

    sub SeeYa { warn"Hasta la vista, baby!" }
    $SIG{'TERM'} = SeeYa;
    print "SIGTERM is now $SIG{'TERM'}\n";

    # perl4 prints: SIGTERM is now main'SeeYa
    # perl5 prints: SIGTERM is now main::1 (and warns "Hasta la vista, baby!")

Use B<-w> to catch this one

=item * (Sort Subroutine)

reverse is no longer allowed as the name of a sort subroutine.

    sub reverse{ print "yup "; $a <=> $b }
    print sort reverse (2,1,3);

    # perl4 prints: yup yup 123
    # perl5 prints: 123
    # perl5 warns (if using -w): Ambiguous call resolved as CORE::reverse()

=item * warn() won't let you specify a filehandle.

Although it _always_ printed to STDERR, warn() would let you specify a
filehandle in perl4.  With perl5 it does not.

    warn STDERR "Foo!";

    # perl4 prints: Foo!
    # perl5 prints: String found where operator expected

=back

=head2 OS Traps

=over 5

=item * (SysV)

Under HPUX, and some other SysV OSes, one had to reset any signal handler,
within  the signal handler function, each time a signal was handled with
perl4.  With perl5, the reset is now done correctly.  Any code relying
on the handler _not_ being reset will have to be reworked.

Since version 5.002, Perl uses sigaction() under SysV.

    sub gotit {
        print "Got @_... ";
    }
    $SIG{'INT'} = 'gotit';

    $| = 1;
    $pid = fork;
    if ($pid) {
        kill('INT', $pid);
        sleep(1);
        kill('INT', $pid);
    } else {
        while (1) {sleep(10);}
    }

    # perl4 (HPUX) prints: Got INT...
    # perl5 (HPUX) prints: Got INT... Got INT...

=item * (SysV)

Under SysV OSes, C<seek()> on a file opened to append C<<< >> >>> now does
the right thing w.r.t. the fopen() manpage. e.g., - When a file is opened
for append,  it  is  impossible to overwrite information already in
the file.

    open(TEST,">>seek.test");
    $start = tell TEST ;
    foreach(1 .. 9){
        print TEST "$_ ";
    }
    $end = tell TEST ;
    seek(TEST,$start,0);
    print TEST "18 characters here";

    # perl4 (solaris) seek.test has: 18 characters here
    # perl5 (solaris) seek.test has: 1 2 3 4 5 6 7 8 9 18 characters here



=back

=head2 Interpolation Traps

Perl4-to-Perl5 traps having to do with how things get interpolated
within certain expressions, statements, contexts, or whatever.

=over 5

=item * Interpolation

@ now always interpolates an array in double-quotish strings.

    print "To: [e-mail]\n";

    # perl4 prints: To:[e-mail]
    # perl < 5.6.1, error : In string, @somewhere now must be written as \@somewhere
    # perl >= 5.6.1, warning : Possible unintended interpolation of @somewhere in string

=item * Interpolation

Double-quoted strings may no longer end with an unescaped $ or @.

    $foo = "foo$";
    $bar = "bar@";
    print "foo is $foo, bar is $bar\n";

    # perl4 prints: foo is foo$, bar is bar@
    # perl5 errors: Final $ should be \$ or $name

Note: perl5 DOES NOT error on the terminating @ in $bar

=item * Interpolation

Perl now sometimes evaluates arbitrary expressions inside braces that occur
within double quotes (usually when the opening brace is preceded by C<$>
or C<@>).

    @www = "buz";
    $foo = "foo";
    $bar = "bar";
    sub foo { return "bar" };
    print "|@{w.w.w}|${main'foo}|";

    # perl4 prints: |@{w.w.w}|foo|
    # perl5 prints: |buz|bar|

Note that you can C<use strict;> to ward off such trappiness under perl5.

=item * Interpolation

The construct "this is $$x" used to interpolate the pid at that point, but
now tries to dereference $x.  C<$$> by itself still works fine, however.

    $s = "a reference";
    $x = *s;
    print "this is $$x\n";

    # perl4 prints: this is XXXx   (XXX is the current pid)
    # perl5 prints: this is a reference

=item * Interpolation

Creation of hashes on the fly with C<eval "EXPR"> now requires either both
C<$>'s to be protected in the specification of the hash name, or both curlies
to be protected.  If both curlies are protected, the result will be compatible
with perl4 and perl5.  This is a very common practice, and should be changed
to use the block form of C<eval{}>  if possible.

    $hashname = "foobar";
    $key = "baz";
    $value = 1234;
    eval "\$$hashname{'$key'} = q|$value|";
    (defined($foobar{'baz'})) ?  (print "Yup") : (print "Nope");

    # perl4 prints: Yup
    # perl5 prints: Nope

Changing

    eval "\$$hashname{'$key'} = q|$value|";

to

    eval "\$\$hashname{'$key'} = q|$value|";

causes the following result:

    # perl4 prints: Nope
    # perl5 prints: Yup

or, changing to

    eval "\$$hashname\{'$key'\} = q|$value|";

causes the following result:

    # perl4 prints: Yup
    # perl5 prints: Yup
    # and is compatible for both versions


=item * Interpolation

perl4 programs which unconsciously rely on the bugs in earlier perl versions.

    perl -e '$bar=q/not/; print "This is $foo{$bar} perl5"'

    # perl4 prints: This is not perl5
    # perl5 prints: This is perl5

=item * Interpolation

You also have to be careful about array references.

    print "$foo{"

    perl 4 prints: {
    perl 5 prints: syntax error

=item * Interpolation

Similarly, watch out for:

    $foo = "baz";
    print "\$$foo{bar}\n";

    # perl4 prints: $baz{bar}
    # perl5 prints: $

Perl 5 is looking for C<$foo{bar}> which doesn't exist, but perl 4 is
happy just to expand $foo to "baz" by itself.  Watch out for this
especially in C<eval>'s.

=item * Interpolation

C<qq()> string passed to C<eval>

    eval qq(
        foreach \$y (keys %\$x\) {
            \$count++;
        }
    );

    # perl4 runs this ok
    # perl5 prints: Can't find string terminator ")"

=back

=head2 DBM Traps

General DBM traps.

=over 5

=item * DBM

Existing dbm databases created under perl4 (or any other dbm/ndbm tool)
may cause the same script, run under perl5, to fail.  The build of perl5
must have been linked with the same dbm/ndbm as the default for C<dbmopen()>
to function properly without C<tie>'ing to an extension dbm implementation.

    dbmopen (%dbm, "file", undef);
    print "ok\n";

    # perl4 prints: ok
    # perl5 prints: ok (IFF linked with -ldbm or -lndbm)


=item * DBM

Existing dbm databases created under perl4 (or any other dbm/ndbm tool)
may cause the same script, run under perl5, to fail.  The error generated
when exceeding the limit on the key/value size will cause perl5 to exit
immediately.

    dbmopen(DB, "testdb",0600) || die "couldn't open db! $!";
    $DB{'trap'} = "x" x 1024;  # value too large for most dbm/ndbm
    print "YUP\n";

    # perl4 prints:
    dbm store returned -1, errno 28, key "trap" at - line 3.
    YUP

    # perl5 prints:
    dbm store returned -1, errno 28, key "trap" at - line 3.

=back

=head2 Unclassified Traps

Everything else.

=over 5

=item * C<require>/C<do> trap using returned value

If the file doit.pl has:

    sub foo {
        $rc = do "./do.pl";
        return 8;
    }
    print &foo, "\n";

And the do.pl file has the following single line:

    return 3;

Running doit.pl gives the following:

    # perl 4 prints: 3 (aborts the subroutine early)
    # perl 5 prints: 8

Same behavior if you replace C<do> with C<require>.

=item * C<split> on empty string with LIMIT specified

    $string = '';
    @list = split(/foo/, $string, 2)

Perl4 returns a one element list containing the empty string but Perl5
returns an empty list.

=back

As always, if any of these are ever officially declared as bugs,
they'll be fixed and removed.