Setting $> seems to work fine on my AIX box (5.2, perl 5.8.8) if I set it to -2 or 4294967294. If I then touch a file it is owned by "nobody" in either case. The comparison fails for -2, but 4294967294 works fine. It looks like the $> variable is getting converted to an unsigned value, which would make sense since the underlying data type uid_t is unsigned.

FWIW, I've had strange problems with setting uid/etc with variables in AIX, at least in the past. You may want to look at the various POSIX routines for this. AIX just loves to be different sometimes...