I noticed that LWP::UserAgent::WithCache handles the 'If-Modified-Since' header. Also, the cache options includes 'default_expires_in'. However, all of this is in terms of time rather than in terms of whether things have changed alone, as would be the case with an HTTP response header line:

Cache-Control: max-age=0, private, must-revalidate

In other words, if all you care about is if something has changed (isn't that really what cache is supposed to be about anyway?) it isn't obvious the LWP caching modules in CPAN do what you want. Am I missing something?