I agree with moritz that there are many that require very little effort and pay handsomely. I interpret your question to also mean, what requires a lot of effort but ends up worth it in the end. These are pretty web centric but I recommend XML::LibXML, HTTP::Request, Catalyst, and any RFC/Standards based modules like URI or JSON. Modules like these, the latter or HTTP::Request (and HTTP::Response) especially, have the wonderful side-effect of teaching you how the underlying standards and technologies work. When you pick those up you're also picking up loads of extra knowledge which will transfer to other areas of work and even other languages.