LWP::Parallel::UserAgent might help you out.

For a quickstart, Apache comes with a simple benchmarking utility called ab. You can use that to do a number of concurrent requests, like ab -c 99 -n 500 http://your.server/url.
ab is not very useful if you need detailed information about the responses on the client side. I use it mainly to see how the server behaves to many concurrent requests.