I just gotta say that Matthias Neeracher is way cool. GUSI is cool. MacPerl supports a lot of POSIX calls very well without much or any additional code, which is just great. Win32 seems to have worse POSIX support underneath, despite being a more POSIX-ish OS. GUSI is the difference: it is a very good POSIX library for Mac OS.
MacPerl, and the perl test suite, are a great GUSI test suite. We've found a lot of little GUSI bugs as MacPerl has moved forward. Each project makes the other better.