in reply to When would you use functional programming?
This isn't really a direct answer to your query but it is related to the intersection between functional programming languages and Perl.
I believe Dominus (aka MJD) is the man who knows most about this (though tilly deserves mention also) from having read his website, Perl Paraphenalia.
I am waiting (but not holding my breath as I would have died by now) for mjd to finish Perl Advanced Techniques Handbook. He does a course on the topic as well.
I think that this book, if it ever comes off the presses would cause a renaissance/revolution in the world of Perl on the order of when thedamian did his book on OO Perl. And, no I don't think that's overstating it.
(If you read this Mark, please take it as tacit encouragement rather than a flame; I fear it will never be possible for me to take your PATH class because I am neither stateside nor working for a potential customer of yours so I have to twiddle my thumbs 'til the book comes.)