http://en.wikipedia.org/wiki/Bloom_filter

http://search.cpan.org/dist/Bloom-Filter/Filter.pm

A Bloom filter is a probabilistic algorithm for doing existence tests in less memory than a full list of keys would require. The tradeoff to using Bloom filters is a certain configurable risk of false positives. This module implements a simple Bloom filter with configurable capacity and false positive rate. Bloom filters were first described in a 1970 paper by Burton Bloom, see http://portal.acm.org/citation.cfm?id=362692&dl=ACM&coll=portal.