As I understand it, MLDBM is just a way of invoking a serializer, it has nothing to do with concurrency. Using flock() or some other locking scheme on the file seems to be the way to go. One option is to use DBD::DBM (part of the DBI distribution), which provides a DBI interface to MLDBM on top of BerkeleyDB or othe DBM file and it provides flocking (on a separate lockfile, not on the DBM file) if you want it.