I have written a brief example for Test::MockObject::Extends
You can see it at: http://logiclab.jira.com/wiki/display/OPEN/Test-MockObject-Extends
It should give you some pointers on how to mock, I will extend the article as I dig up examples from my own code base.
I can really recommend the book: Perl Testing: A Developer's Notebook by Chromatic and Ian Langworth. See: http://oreilly.com/catalog/9780596100926
Articles might also shed some light: http://www.perl.com/pub/a/2005/04/07/mockobject_kata.html