for (0 .. 10) {
    my $outer_elem = $_;
    for (0 .. 20) {
        my $inner_elem = $_;

        do_something($outer_elem, $inner_elem);
    }
}

# Yeah, I know doing this is better:
for my $outer_elem (0 .. 10) {
    for my $inner_elem (0 .. 20) {
        do_something($outer_elem, $inner_elem);
    }
}

##</code><code>##

foreach my $elem1 (@array1) {
    my $outer_idx = $.;
    foreach my $elem2 (@array2) {
        my $inner_idx = $.;
        
        # Blablabla...
    }
}