rolfy has asked for the wisdom of the Perl Monks concerning the following question:

long time reader first time caller....
Anyway, i'm currently upgrading some legacy scripts to perl 5.10. There's a lib we've got that for some reason decided it'd be easier if arrays started at 1, so they've gone and set $[ = 1
They've done this in a module, in an init function. The code has previously run in 5.8.8, and i'm updating everything to use perl 5.10.
my test.pm example: 
#!/usr/bin/env perl

package test;

sub Init
{
    $[ = 1;
}

sub test
{
    print "(\$[= $[) array has " . scalar @_ . " elements, last index 
+= $#_\n";
}

1;

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t1.pl: 
#!/usr/bin/env perl

use test;

test::test(1,2,3,4);

# test::Init();

test::test(1,2,3,4);

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As it turned out, even without calling test::Init, the mere fact that that is in that function has somehow globally turned on that option...
in perl 5.8.8, i execute the t1.pl and get: 
# perl 5.8.8 output
./t1.pl
($[= 1) array has 4 elements, last index = 4
($[= 1) array has 4 elements, last index = 4

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and in perl 5.10 
# perl 5.10
$ ./t1.pl
($[= 0) array has 4 elements, last index = 3
($[= 0) array has 4 elements, last index = 3

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regardless of whether i call Init or not. I'm not sure why this happens, and it looks like it was a bug in perl 5.8.8 that wasn't listed as resolved in perl 5.10 but is...
It drove me crazy :| hopefully this information at least helps someone if they see the same thing - i've not seen it listed anywhere as a change or as something someone has tripped over. Can anyone else confirm this or is it just me? :|

Replies are listed 'Best First'.
Re: fun with $[
by ikegami (Patriarch) on Aug 06, 2008 at 03:44 UTC

    In 5.8.8, it behaves as documented. Quote 5.8.8's perlvar:

    As of release 5 of Perl, assignment to $[ is treated as a compiler directive [...]. (That's why you can only assign compile-time constants to it.)

    In fact, 5.10.0's perlvar still includes that line, so you've uncovered a bug. (Documentation or code bug, I don't know.)

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Re: fun with $[
by parv (Parson) on Aug 06, 2008 at 02:21 UTC
    I see the same behaviour as listed with 5.8.[78] & 5.10.0. Try also Deparse (perl -MO=Deparse file) ... 
    #  test.pm was used in p.pl.
#
#  perl 5.10.0
package test;
sub Init {
    $[ = 1;
    BEGIN {
        $^H{'$['} = q(1);
    }
    $[;
}
sub test {
    $[ = 0;
    print '($[= 0) array has ' . scalar(@_ . " elements, last index = 
+$#_\n");
}
'???';
test.pm syntax OK


use test;
test::test(1, 2, 3, 4);
test::Init();
test::test(1, 2, 3, 4);
p.pl syntax OK

#  perl 5.8.7
package test;
sub Init {
    $[ = 1;

}
sub test {
    print '($[= 1) array has ' . scalar(@_ . " elements, last index = 
+$#_\n");
}
'???';
test.pm syntax OK


use test;
$[ = 1;
test::test(1, 2, 3, 4);
test::Init();
test::test(1, 2, 3, 4);
p.pl syntax OK

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Re: fun with $[
by syphilis (Archbishop) on Aug 06, 2008 at 02:39 UTC
    Of $[, the perlvar documentation says "Its use is highly discouraged".
    And what better way to discourage its use than have it behave in the way you've presented :-)

    Cheers,
    Rob
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Re: fun with $[
by Perlbotics (Archbishop) on Aug 06, 2008 at 14:23 UTC
    Maybe it is an option for you to localise the $[ like so? 
    !/usr/bin/perl
use strict;

package LocalBlock;
{ # localisation scope
  local $[ = 1;

  sub test {   # now a closure
    print "(\$[= $[) array has " . scalar @_ . 
          " elements, last index = $#_ (", 
          join(", ", @_) , ")\n";
  }
} # end of scope
1;

package main;

print "Pre:  \$\[ = $[ \n";
LocalBlock::test(1,2,3,4);
print "Post: \$\[ = $[ \n\n";

$[ = 5;
print "Pre:  \$\[ = $[ \n";
LocalBlock::test(5,6,7,8);
print "Post: \$\[ = $[ \n\n";

__END__
Pre:  $[ = 0
($[= 1) array has 4 elements, last index = 4 (1, 2, 3, 4)
Post: $[ = 0

Pre:  $[ = 5
($[= 1) array has 4 elements, last index = 4 (5, 6, 7, 8)
Post: $[ = 5

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    Cannot test it without 5.10 here, sorry.
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Re: fun with $[
by JadeNB (Chaplain) on Aug 20, 2008 at 21:05 UTC
    I don't understand why it would behave differently in Perl 5.8.8, but the following paragraph from perldata seems consistent with the Perl 5.10 behaviour you've described:
    Version 5 of Perl changed the semantics of $[: files that don't set the value of $[ no longer need to worry about whether another file changed its value. (In other words, use of $[ is deprecated.) So in general you can assume that 
    scalar(@whatever) == $#whatever + 1;

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    (I know this is an old question by now, but I was looking for this line that I vaguely remembered in perldelta, not perldata.)
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