As far as I know, Perl on Linux, BSD and OS X systems will happily allocate memory it gets terminated by the scheduler after nearing 100% usage. I have 1.5 GB of RAM and have had perl scripts go up to 90% memory usage without requiring any special command line switch.
Could you provide more information about your operating system, and why you aren't getting the memory you need? Part of Perl programming is supposed to be never worrying about memory until you run out, but these issues are heavily OS-dependent.