use strict; helps a whole lot less if you:

my ($i,$j,$tmp);
[download]

and actually we are using Perl so you can:

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

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

for my $i (0 .. $#array) {
    for my $j ($i + 1 .. $#array) {
        @array[$j, $i] = @array[$i, $j] if $array[$j] < $array[$i];
    }
}

print "array : @array \n";
[download]

If you'd used warnings you'd have caught the error in the process of fixing the warnings.