> I've been looking at Sys::Info::Device::CPU but it's neither portable to windows

Actually it's very portable :) It's part of my Sys::Info module and currently Windows, Linux & (Free|Open|Net)BSD is supported. However, fot the windows part the cpu load thing is returned as the current CPU load "ratio". The value you can see in the task manager, which is different than the linuc loadavg. So, it's only an emulation on windows (however, if there is a better way, please send a patch :)