If you just type system("notepad $filename"), then the Tk main loop will be held up until the user closes Notepad. This means that the Tk window won't get painted. (At least in all the versions of Tk I've used.) Kinda ugly.

If you want to spawn Notepad but don't want to pause the script waiting for Notepad to close, use Windows' START command:

system("start /b notepad $filename");
[download]

This runs Notepad in the "background" (a misnomer, since it will be activated just fine for the user to see it), and the script will continue immediately.

The Unix tradition is to allow the default editor to be configured, such as with an environment variable. The user can adjust this before running your application, if desired. I would code this like this:

my $editor = $ENV{EDITOR} || 'notepad.exe';
system("start /b $editor $filename;
[download]