use constant N => 3;

sub mmMxM {
    
    my ($ar_A,  # ref. to array of ...
        $ar_B,  # ref. to array of ...
        ) = @_;

    my @c = map [ map 0, 0..N ], 0..N;  # AoA of zeros
    
    for my $i (0..N) {
        for my $j (0..N) {
            $c[$i][$j] += $ar_A->[$i][$_] * $ar_B->[$_][$j] for 0..N;
            }
        }
        
    return \@c;

    }