Why use IO::File

Eh, why not? It's part of Perl CORE, isn't it? I don't deny that there are many other ways to do I/O (with plain open or using IO::Handle for instance), but is there a drawback with IO::File?

Also when you read the input file, the line terminator is still there, and then you're adding another one with your join.

Indeed! I forgot to chomp! Thank you for pointing this out.

is your script running on Windows or Unix?

The program is supposed to be run on either Windows or Unix. I try to make it as independent as possible (I try to avoid coding two variations of the program for each platform). The input file is guaranteed to have 0x0a as line terminator, even when run under Windows.

I actually leave out the closes usually.

Then I would have at least to flush the buffers, otherwise other applications won't be able to access the file, until my program terminates. Also, I think they won't be able to open the file for writing while I still have it open, though this might be platform dependent. In both cases, a close makes sense IMO.