As paulgb said, Lisp is not purely functional, unlike Erlang or Haskell.

And actually, you could ignore functional programming and write code in an imperative style in Lisp (though it's not recommended, since you'd be missing out on the benefits of using Lisp).

Take a look at "Imperative Outside In" (chapter 3, section 2) in "On Lisp" (http://lib.store.yahoo.net/lib/paulgraham/onlisp.pdf) for an example.