Take a look at POE, in particular POE::Component::Client::HTTP. From the POD of the latter:

POE::Component::Client::HTTP is an HTTP user-agent for POE. It lets other sessions run while HTTP transactions are being processed, and it lets several HTTP transactions be processed in parallel.

I've not used it personally, but it would seem to allow all you need.

LWP::Parallel might also be of use.