There is a Statistics::Distributions module; but I'm sure you know about that.
I'm a bit confused by your post (I'm easily confused, so don't be offended), in that you're saying you've got the distribution, the mean, the standard deviation, and a target percentage. Isn't this overspecified? Or do you want to be able to get z given a target percentage and vice versa?
Scanning CPAN does seem to indicate you'll have to roll your own; you might have success at http://lib.stat.cmu.edu, but you've probably already looked there.
Update: One of thor's replies unconfused (or, at least, I think it did) me; Statistics::Distributions gives n = F(z), but he needs the inverse of this. i.e., z = F^{1}(f).
