Tie::FileLRUCache provides a persistent filesystem based LRU cache. It uses the 'last accessed' timestamp generated by the file system to determine the 'oldest' cache entry and discards the oldest cache entries when needed to stay under the -keep_last limit. If you store thing very fast (such that many entries receive the _same_ time stamp), it is essentially a coin toss which entry within a single timestamped second gets purged from the cache to make room for new ones.