The short answer is "not very easily". If you look in perlform, you can get some ideas from the "Footers" section. Basically you'll have to keep track of $- (aka $FORMAT_LINES_LEFT if you use English;) and do something different when it hits zero.

In general, formats are best for fixed-column output. HTML will work, but will take a lot of work. I have to echo the sentiment above on using a templating system.