sub cartesian {
    my @sets = @_;
    # base case
    if (@sets == 0) {
        return ([]);
    }
    my @first = @{@sets[0]};
    # recursive call
    shift @sets;
    my @rest = cartesian(@sets);
    my @result = ();
    foreach my $element (@first) { 
        foreach my $product (@rest) { 
            my @newSet = @{$product};
            unshift (@newSet, $element);
            push (@result, \@newSet);
        }
    }
    return @result;
}