This isn't really a Perl question as much as a HTML question, is it? Anyhow, prepopulating text boxes with default values is as easy as outputting

<input type="text" value="something">
[download]

As before, the the MDN has more. In fact, it has a lot of information on all sorts of web-related technologies, from HTML to CSS, from DOM to Javascript, and more; I really recommend bookmarking it and referring to it. It's an invaluable resource.