Your selected algorithm is the best way to control Perl's memory usage.

First, I might suggest that you decode the "wierd" date in your file ONE time, by going through the large file once, and rewriting it to a new file with the "proper" date.

Second, if your Perl program is just a sorting thing, (or that is at least a major function of it), then if it's a big enough problem, purchasing a dedicated specialized sort program for your OS might be a better investment. Syncsort is such a product that may fit your needs. There are versions for Windows and for most important flavors of UNIX.