Chapter 14 in Perl Cookbook, by Tom Christiansen and Nathan Torkington (O'Reilly)
Chapter 17 in Learning Perl, Second Edition, by Randal L. Schwartz and Tom Christiansen (O'Reilly)
Chapter 2 in Programming the Perl DBI, by Alligator Descartes and Tim Bunce (O'Reilly)
The Berkeley DB web site: http://www.sleepycat.com/