I think a language that _supports_ functional programming like Lisp or Perl is not well-suited because albeit they contain the paradigm, you are the one to put the limits. And you can't do that very well if you don't know the paradigm in the first place.

I mean, we have closures, we have recursion, we have first-order functions, map, the possibility to define composition, currying, ..., but you need to learn to think functionally using that (pretty small) subset of the languages. You need to consciously leave the rest out.

If you take Haskell everything is easier, the language forces you to think that way.