Personally, my preferred method would be to:

1. Refactor it to no longer be standalone, but an API that others could use and embed into their own Tk applications.
2. Modify the script so that it used the API to create the TkMailer window to send whatever you already do.
3. Put this whole package on CPAN.

You can ship scripts with your modules on CPAN. The focus on the modules, however, is a focus that allows for code-reuse in other applications, which is one of the strengths of perl and CPAN, IMO.

How to on packaging: check ExtUtils::MakeMaker or Module::Build - I think they both talk about how to use them to package your modules.

How to classify: you can ask here. I've seen that done a few times. To give you a headstart, if you do as I suggested above, I'd classify it under "User Interfaces" since it'll be tied closer to Tk than to SMTP.

Who defines - we all do, but if you check PAUSE, they have a section on this.

Hope that helps. It's actually a lot less daunting than it looks. Especially the second time. ;-)