I think it's a fine idea, as long as you're reasonably sure you're the only one that will ever need to edit the HTML. In fact, this is pretty much exactly what I did in HTML::PopupTreeSelect. I used a here-doc rather than DATA but the effect is the same.

-sam

PS: Ignore anyone that suggests you switch to another templating system. HTML::Template has always served me well and I expect it will continue to do the same for you!