Regexp::Debugger does a great job of illustrating how the regex engine actually does matching. It can help visualize the execution, helping programmers see and fix inefficiencies. For programmers new to regex, it's a great learning tool too.
This is a much nicer alternative to Test::Mock::LWP, but the mock object exported by default doesn't pass the isa test. Instead, you need to create your own object - I've explained more fully at hashbang.ca/2011/09/23/mocking-lwpuse..., and submitted a patch that should fix this issue.
MediaWiki::API provides convenience methods for accessing the modern MediaWiki API at a low level. This is incredibly useful, but lacks the high-level abstraction necessary to maximize programmer productivity. It is used internally in MediaWiki::Bot, which provides much of that higher level of abstraction.
The newly-released version has fixes for the huge bug mentioned above: it works with :encoding by passing it off to binmode. Huzzah, File::Slurp is finally worth using! Many thanks to the new maintainer Uri Guttman for picking through the patches on RT and rolling them into the latest release. I hear good things for the future too!