I want to open a gdbm file.

From the documentation, it appears that the DB_File module cannot be used with gdbm files:

DB_HASH

This database type allows arbitrary key/value pairs to be stored in data files. This is equivalent to the functionality provided by other hashing packages like DBM, NDBM, ODBM, GDBM, and SDBM. Remember though, the files created using DB_HASH are not compatible with any of the other packages mentioned.

Try the GDBM_File module instead.

Hope that helps,