Erm, newgrp creates a new shell environment with the changed group credentials; that's not going to do your parent process much good. You need to look at $) and friends in perlvar if you want to change things for the current process.

Update: Duuur, quite right. You'd have to be privileged to muck with $) which you're not because you're resorting to newgrp. Machinations like this have to be done outside your code itself (either by the running user using newgrp then running your code, by having your code setgid as is mentioned below, or by having a small setuid wrapper use $) on your behalf then running your program).