According to the documentation of that module it won't work on the current YouTube API. I think what it's looking for is a developer key which you'd get from Google if you were developing an application that made use of the YouTube API. If you just want something that'll download videos this guy demonstrates how that can be done using WWW::Mechanize and provides a short script. He also mentions that the API changes fairly regularly and provides an updated script towards the end of his post to accommodate changes since he originally blogged it.