Very strange and dangerous... Some workarounds:

use strict;
use warnings;
use bigint;

print "\nWith two foreach loops:\n\n";

for my $j (20 .. 22)
{
    for my $i (12 .. 14)
    {
        print $i ** ($j+0), "\n";
    }
}

print "\nWith two foreach loops:\n\n";

for my $j (20 .. 22)
{
    for my $i (12 .. 14)
    {
        print (((0+$i) ** $j), "\n");
    }
}

print "\nWith two foreach loops:\n\n";

for my $j (20 .. 22)
{
    for my $i (12 .. 14)
    {
        print ((0+$i) ** ($j+0), "\n");
    }
}
[download]