#!/usr/bin/perl
use warnings;
use strict;

my @array = (12, 6, 2, 9, 15);

for (my $i = 0; $i < $#array; $i++) {
    for (my $j = 0; $j < $#array; $j++) {
        if ($array[$j] > $array[$j+1]) {
            my $tmp = $array[$j+1];
            $array[$j+1] = $array[$j];
            $array[$j] = $tmp;
            warn "$i $j : @array\n";
        }
    }
}

print "array : @array \n";
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Inserting the debugging warn into your original code, you can see the array's getting longer and longer, as $array[$j+1] fetches an undef from the yet non-existent element after the end of the array (warnings omitted):

0 0 : 6 12 2 9 15
0 1 : 6 2 12 9 15
0 2 : 6 2 9 12 15
0 4 : 6 2 9 12  15
0 5 : 6 2 9 12   15
0 6 : 6 2 9 12    15
0 7 : 6 2 9 12     15
0 8 : 6 2 9 12      15
0 9 : 6 2 9 12       15
0 10 : 6 2 9 12        15
0 11 : 6 2 9 12         15
0 12 : 6 2 9 12          15
0 13 : 6 2 9 12           15
0 14 : 6 2 9 12            15
0 15 : 6 2 9 12             15
0 16 : 6 2 9 12              15
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Update #2: BTW, do you know that in Perl, you can swap two variables without an explicit temp?

    ($x, $y) = ($y, $x);
    @array[$j, $j+1] = @array[$j+1, $j];
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Update: added explanation.

($q=q:Sq=~/;[c](.)(.)/;chr(-||-|5+lengthSq)`"S|oS2"`map{chr |+ord
}map{substrSq`S_+|`|}3E|-|`7**2-3:)=~y+S|`+$1,++print+eval$q,q,a,
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