Using a larger buffer size may increase throughput slightly, but then it may not. It will depend upon many factors mostly to do with your file system buffering, disk subsystems etc. The easy answer, given it's only taking 10 minutes is to try it.

As far as using threads to distribute the load across your processors is concerned, it certainly could be done, and could in theory, give you near linear reductions per extra processor.

But, and it's big 'but', how good is your runtime library's handling of multi-threaded IO to a single file?

On my system, even using sys* IO calls and careful locking to ensure that only one thread can seek&read or seek&write at a time, something, somewhere is getting confused and the file is corrupted. I suspect that even after a syswrite completes, the data is not yet fully flushed to disk before the next seek&write cycle starts.

So, maybe you could get it to work on your system, but I haven't succeeded on mine, and I am not yet entirely sure whether the problem lies within Perl, the OS, or some combination of the two.

If you feel like trying this, please come back and report your findings. If you need a starting point, /msg me and I can let you have my failing code.