ikegami,
Good catch on s/scalar/string/. I meant to add a line indicating that you could actually use just about anything you wanted as a hash key but that it would end up being stringified and unlikely do what you wanted.

Regarding the luxury of using regexes - if your problem space fits a very specific model, you can still avoid the O(N) by using the technique outlined at Re: Massive regexp search and replace. With this particular implementation, I originally assumed that the idea was to loop over the regexes as you indicate with O(N) but then the usage in the example implied they index would be known. I probably read too much into it. Update: I should have said - either you are limited to integer dispatch values for O(1) or you will have to loop over the entire list O(N).