20mbs, even with a few copies in hashes and arrays here and there, probably fits in memory without any problem. There is some overhead in perl variables and more in the more complex structures (hashes and arrays) but even so you should be OK unless you have very limited virtual memory available. I would keep everything in memory (from perl's perspective) and let the swapper deal with disk if necessary, unless this proved to be problematic. You might have a look to see how much free RAM is available on your system.