From README:

This module automatically inserts data from a previous HTML form into
the HTML input and select tags. It is a subclass of HTML::Parser and
uses it to parse the HTML and insert the values into the form tags.

One useful application is after a user submits an HTML form without
filling out required field.  HTML::FillInForm can be used to redisplay
the HTML form with all the form elements containing the submitted
info.