This isn't a very perl answer, but.....

You you can use the ab program that comes with Apache for benchmarking/load testing a single .cgi (it can be used to test any HTTP server, not just apache). Very good tool that can test multiple simultaneous requests, etc....

If you want to test a more realistic load, a simple load generator can be written using the LWP module that will hit multiple pages on your server. Then you can fire off multiple copies of this script until you discover the magic number of simultaneous clients it takes to saturate the load on your server.

Its also useful to use tools like top to look at the server load statistics (memory vs swap, IO, CPU utilization and queue, which processes are using the most resources, etc...) while you're benchmarking to find the server's bottlenecks.