400MB is not a “very large” file. It is “not a very large file at all.”

What you need to do is use a disk-based sort. There are many of these in CPAN, such as File::Sort. Or, how aboutSort::External? As it happens, “sorting” is one of the most extensively-studied and extensively-implemented algorithms in all of computer science. (The other is “searching,” saith the august Dr. Knuth.) My CPAN-search found 1,929 choices under “Sort.”

Although virtual-memory sorting is simple and easy to conceptualize, it is not intended for large volumes of data. You're seeing it choke, and fail, trying to do a task that is much larger than it was designed for.

Disk-oriented tools, of which there are a great many available, will automatically handle the job by splitting the total job into a group of disk-based spill files, then merging those files together for you. An arbitrary amount of data can be sorted in this way and it will be quite fast.

(Heck, you can even sort data using nothing more than magnetic tape! When all those campy old sci-fi movies needed to film “a computer,” the operators would obligingly start a tape-sort.)