Ecellent package - no need to do the open("| /usr/lib/sendmail -t") thing ever again.
And if you want to send cool HTML mail with text alternative, no problem, this module takes care for everything!
HTTP::WebTest is an excellent choice if you are looking for a flexible end-to-end web(-site/-application) test suite.
The numerous modules give you a rich suite of test possibilities, which are scriptable from a simple test script language.
Advanced tests can be written by embedding perl code in test scripts or choosing one of the many extra test plugins.
All this flexibility comes at the price of not being very fast, but that hardly counts for most monitoring applications I can think of.
See HTTP::WebTest::Cookbook for a quick start!