To be honest, I have never written a program of an signifagance in any purely functional language. I have however read a number of books on a number of functional languages, which has helped me in my day-to-day perl programming. But much of my motivation to learn functional programming was purely curiosity driven (actually boredom driven, I was doing a lot of javascript at the time). My suggestion is not to worry so much about the language you choose to read about/experiment with, but to concentrate on the concepts behind those languages, and let them inform your day to day programming (in perl or whatever). So much of what makes functional programming so cool is not language specific, and a lot of it can be done in perl (5 and 6).