OK, AppleFritter, thanks for your answer.

Thinking more about it, what I get with this program is actually 27 permutations, not 27 combinations. But English is not my mother tongue (and, as far as I can say, probably also not yours), so I probably got a bit confused about it. And I did not know anything about multisets before (or had forgotten everything about it).

Thanks a lot for the clarification.

I haven't checked thoroughly, but it seems that the program I have suggested in my other post (http://www.perlmonks.org/?node_id=1198576) probably does what you want.

Update: And, BTW, my code in the other post runs in less than 1/20th of a second, so it is also fairly fast.