Package::Require caches files that it has already required or failed to require for a speed increase.

How often do you require non-existent modules? Let alone multiple times? I understand the occasional need to do a require, then maybe fall back on some alternate behaviour if the module is not installed, but once the require fails, I don't think I would try to require the same module again... unless I thought the module may have been installed after the last failure, in which case I wouldn't want to cache the failure. (update: In any case, caching the failure seems like a premature optimization which wouldn't increase speed much, if any, and caching success wouldn't save anything).