You are using <INP> in list context. That will make Perl slurp in the entire file, and turn it into a list of lines.

The standard idiom of processing a file line by line is:

    while (<IN>) {
        # Processing goes here; the current line is in $_
    }
[download]

That should cut your memory usage as it doesn't read in the entire file in memory (unless there's just one line in the file...)

Abigail