Bit of a shame that we currently don't have a working YouTube download script/module (WWW::YouTube::Download is last updated 2013 and has been broken for a long while). This module actually requires another *Python* script to do its job. I might as well skip this and go straight to the Python script.
UPDATE 2016-03-04: I guess it's been so for a few years, but this still needs to be said: For downloading YouTube videos, use youtube-dl (a far more popular Python project) and just forget the rest. Keeping up with YouTube changes is many times a full time job. Nothing else comes remotely close.
This module is based on WWW::YouTube::Download but its documentation does not yet explain how it differs from WWW::YouTube::Download. From what I see at a glance, App::YTDL supports downloading a video from a playlist and setting download speed limit, but perhaps the author should do the mode detailed explaining to help users when to choose between the two.