While I agree whith the "whatever works for you" philosophy, it's probably worth asking him if he wants an IDE just because that's what he's used to.

If he's never tried living inside emacs (my favourite) or vim + shell it might be worth his while trying it out. If someone can use a "project search" function of an IDE but has never learned to use grep is going to miss out when he tries to grep log files etc.

OTOH, if you are always a vim + shell person it may be worth *your* while trying out a more interactive environment. I especially love the interaction between emacs and the perl debugger - I assume the EPIC or ActivePerl debuggers would be even better.