Most dbms don't flush writes until you close them. That means you must close and reopen GDBM_File every time. The module MLDBM::Sync will do this for you with a variety of dbm implementations.

The BerkeleyDB approach should be much higher performance. Using database-wide locks instead of block-level locks usually helps with locking issues and doesn't slow it down much. I don't remember the flags to specify this or I'd give them to you. You might also try checking Oracle's support pages.