Any solution which applied to the database as a whole (such as encrypting the entire file, and decrypting it at program start) would be vulnerable in that someone else could read the data while your program ran.

It sounds as if the simple solution would be to encrypt the data going into the database, and decrypt it on retrieval. Crypt::xDBM_File will do that for you. You'll also need an acceptable encryption module such as Crypt::DES.

A simpler solution for this would be to use strict file protection, if the program is run by a single user, or to move the data to a database such as MySQL which provides built-in password protection.