I was wondering why you made such a point of mentioning mod_perl. The way Perl is executed has nothing to do with the rest of your question, AFAIK.

SHM -- which is still SYSV :) -- works perfectly, and you can access it with no fear as long as it's Write Once, Read Many blocks. There's no need to use a module at all in this case. Even access for others to write is not difficult; you just build a common lock-block of bits (or bytes, if you can afford to be wasteful), which acts like a set of semaphores. No more tricky than file locking.

If you intend Net-wide access, think again about databases. Your CPU + IO time will still be far less than the transmission time.