Hi, I was just thinking what a nice module to write. What should I name it? Did a search on 'System' in CPAN and found these modules: Sys::CpuLoad, Sys::Load and System::Index (for a moment I'll not distinguish between CPU usage and 'load').
System Index has an original approach of having some fixed functions by which an index for memory, CPU and disk-usage is calculated. This might tell you more than CPU-usage or load (I'm presuming here you want to know more about system performance, not only CPU-usage). I've seen machines with loads over 200 that had no performance problems (lots of processes in the run-queue that only needed a short slice of processor time). CPU usage will tell you something about CPU usage (duh), I mainly use that to estimate if the CPU is a bottleneck (if usage is >80% most of the time).
My point: If somebody is 2.20 meters long that is an indication that he/she might be a good basketball player, but you have to take into account a lot more then heigth alone.

update: fixed the System::Index link, I can't find it via search.cpan.org, only via kobesearch.cpan.org