I think that that 'someone' will have to learn the basics of your templating engine, if she is to maintain the web site without your further involvement as a web service programmer. If she is to be in control of things, then I suggest that you develop an API-like system: for every template define bunch of variables of the system, that the designer can work with. You write the docs about the meaning of each variable and present them to the designer. She is then able to change the presentation as she likes.
I am in somewhat similar position; I am the web design maintainer and the programmer on the server side (I got finished design of our site). And I did just what I described: so if I want to change something I don't have to browse through server-side source, my template already 'knows' a lot.
Also, if you plan some serious programming stuff on the template side, I suggest you try
Template::Toolkit as
HTML::Template is somewhat limited to simple expressions.