sub mmMxN {
    our( @M, @N );
    local( *M, *N ) = @_;
    die "Incompatible matrix dimensions" unless @M == @{ $N[0] };

    my @C = map[ (0) x @M ], 0 .. $#{ $N[ 0 ] };

    for my $i ( 0 .. $#M ) {
        for my $j ( 0 .. $#{ $N[0] } ) {
            $C[ $i ][ $j ] += $M[ $i ][ $_ ] * $N[ $_ ][ $j]  for 0 .. $#N;
        }
    }
    return \@C;
}