The Black Marble Wombat is a simple MLM written in Perl.  It was developed
by Clay Luther of Monsta Incorporated <bmw@gojira.monsta.com> to run some
lists on gaming.  The software is available free under the GNU license.

Unusually, digests are requested through a separate subscribe command (called
DIGEST, logically enough).  Subscribing to a list digest simultaneously
unsubscribes a user from the regular list; to unsubscribe once he or she is
on the digest list, the user uses the UNDIGEST command.  BMW optionally
archives messages on a monthly basis.  Beyond that, BMW is pretty minimalist:
available user features are automatic subscription and unsubscription,
archive sending and listing, and list review.

Pick BMW up at <ftp://gojira.monsta.com/pub/src/>.