Perl Testing: A Developer's Notebook is an excellent book. It's a pleasure to read and full of examples. I hadn't written a single test before reading that book - now I'm hooked. :-)
Test::Tutorial is a good tutorial and a great place to start.
Test::Simple and Test::More are two modules that provide a lot of basic testing capability.
Search the Test namespace on CPAN - there are a lot of other testing modules available.
Don't forget to Super Search PerlMonks. Here is a sampling of some recent threads on testing:
Snippets of code should be wrapped in
<code> tags not<pre> tags. In fact, <pre>
tags should generally be avoided. If they must
be used, extreme care should be
taken to ensure that their contents do not
have long lines (<70 chars), in order to prevent
horizontal scrolling (and possible janitor
intervention).