I don't think a compile time switch is a good solution, that's far too restricitve. Especially since quite a few people do not compile their perl!

Besides what you say assumes that it's always OK to use up all the power of the machine and also it's the way you would want to use the paralelization for the computationaly expensive tasks. In case the operation you want to do with the element of the list spends more time waiting for some data then you will want to use more threads. Eg. if you wanted to test the conectivity to the computers specified in the list you will spend most time waiting for the pings to return or time out, not computing anything. So you would want to create quite a few more threads than if you just need to use all the CPUs your box has.