I should know better then to answer these when I'm tired.

As he has permission to install his program and Perl with it, I assume (we all know where that can lead) I hope he has the smarts to ask the end user first if its ok to make any changes. But a better solution would be to do the following

system ('start c:\windows\notepad.exe filename.txt');
[download]

Now he will always get Notepad to open the text file.