sub matrix_ok {
    my ($matrix) = @_;

    {
        my $i = 4;

        for my $j (0 .. 2, 4 .. 6) {
            for my $k (0 .. 2) {
                return 0 unless $matrix->[$i][$j][$k] == 1;
            }
        }
    }
    {
        my $j = 4;

        for my $i (0 .. 2, 5, 6) {
            for my $k (0 .. 2) {
                return 0 unless $matrix->[$i][$j][$k] == 1;
            }
        }
    }

    return 1;
}