What about keeping a tied hash (DB_File, SDBM, GDBM or whatever) of the files you've "seen" and

• if you encounter a file you did not see yet (it's not in the hash):
  • add it to the hash
  • change it's modification time to the current time
• if you encounter a file that is in the hash and is older than 30 days:
  • delete it from the disk
  • delete it from the hash

Jenda