All your points are good, I'll look into LWP callbacks so I can avoid HEAD requests - the mixing of decoded_content and content_file was due to me misunderstanding what decoded_content did. Can you elaborate what it does a little bit, for me?
Comment on Re^2: RFC: Code testers/reviewers needed
Documentation for decoded_content: http://search.cpan.org/perldoc?HTTP%3A%3AMessage
Note that there is no content at all, as file downloaded using content_file option.