my @arr = ([1,2],[3,4]);

for (@arr) {
    for (@$_) {
        print "$_\n";
    }
}
[download]

EDIT: Or if you want to keep track of where you are:

use strict;
use warnings;

my ($x, $y);
my @arr = ([1,2],[3,4]);

for $x (0..$#arr) {
    for $y (0..$#{$arr[$x]}) {
        print "\$arr[$x][$y] = $arr[$x][$y]\n";
    }
}
[download]