The answer by ikegami is a good starting point, as those modules will get you both MIME e-mail creation and a templating solution.

The most direct answer to your question is that you or someone will have to modify the contents of the sendemail function that your snippet shows being invoked. Since senemail is not part of the Perl core language or of the standard module set shipped with Perl, it must have been written at the same time as your application. That's the modify point you need, where you will be adding code to use Text::Template, HTML::Template, or Template::Toolkit to have templates that will be turned into MIME attachments by MIME::Lite