I'm pretty sure (not positive, so someone please correct me if I'm wrong) that rand() has system-specific behaviour, and that if you seed it on a Windows machine, you could get a different sequence from the exact same seed on a BSD machine. This is fine if it's in-house, but if you ever want to be able to say "hey - found a cool sequence - try out seed xxxxx", they might not get the same sequence.

I'm not sure if there's a pure-perl PNRG on CPAN somewhere, but when I came across the problem in a C project, I ended up writing my own. It's a relatively simple algorithm, and a quick google shows up a number of results.