After my endless testing and bugswatting, XML::RSS::TimingBot is basically useable now. It's an LWP::UserAgent subclass that stops you from requesting RSS feeds more often than they say they'll update. I.e., when you request a feed, the module transparently looks for itselements and the like, and the next time you request it, if the previously-read information says that the feed won't updated by now, the module won't actually perform the request, but will generate a virtual response that looks like the server said "304 Not Modified" with no content. It also keeps track of last-modified times on RSS responses and inserts "If-Modified-Since" headers on subsequent outgoing responses. It's all very behind-the-scenes and transparent.
RSS フィードの syndication モジュールとかをつかってリクエスト周期を変更するモジュールかな？ もうすぐCPAN にリリースされるとか。楽しみです。