Well I don't think I need to tell you about "Programming the Perl DBI" as it's been plastered in all of the other replies.

I've yet to see a book on PostgreSQL, but there's a MySQL book that has Perl DBI examples and teaches you MySQL at the same time. It also has examples for other languages which I don't feel I need to stress.

MySQL by Paul DuBois. New Riders is the publisher.