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 over-specified? 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 un-confused (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).