There's fam which is available on several variants of *NIX, and SGI::FAM (which looks to be a little long in the tooth). I want to say the BSDs were moving in a similar direction with some sort of kernel event mechanism (IO::KQueue looks to be the Perl interface to that).

Also a suggestion for future questions: Asking about "UNIX" in this general manner is about like going into an auto parts shop saying you need a part for a "vehicle" and expecting to get the right one for your make and model of car; it leaves so much wiggle room that the only sure answer is "probably" and guarantees vague hand waving. (Unless of course you are interested in *NIX in general in the sense of a survey of what's available where, but even then it doesn't hurt to make that fact explicit :)