'multimap' may not be the best name for that function..

I just went with the OPs name, but I agree that it may not best capture the semantics. I'd favour something shorter. 'map' is well understood in Perl circles so I think that should be in there somewhere.

Yeah, I was at a loss as to what to name this thing. In Scheme and other Lisp dialects, it's just called map. I am aware that multimap clashes with part of the C++ STL, but then again, so does map. Maybe mapmulti is a good name.