I think that your problem is different. We already have a caching mechanism with LWP::Simple::mirror. What you are asking for is something that will defeat the purpose of HEAD method for something already stored somewhere and that will not obey no-cache instructions. This is reasonable, especially when you don't want to overload an external resource. I thought many times about this kind of caches and tried few times to write a module.

I came up with a 'not_before sometime' policy that is well is applicable to Cache::Cache mentioned by PodMaster; furthermore that module can be used to cache complex URLs. If you are interested I can post some code.

At the other end of the spectrum there are proxies, they are built to do this kind of work.

Ciao, Valerio