I've done similar stuff using HTML::Template. You make a wrapper template that contains your formatting: styles, div tags, pre tags, etc., and in the middle you have one special template variable intended to hold your ascii art, then you write a cgi script to load the template, slurp in the ascii file, stuff it into the template, then print it out to the browser.

Since templates can contain other templates, this technique works well anytime you want to build a "widget" of some kind (a little block of HTML that isn't really a page by itself, but that may get used over and over).

For example, if you wanted to build a photogallery web page, you could build a little template for formatting a single thumbnail image with a caption centered below it. Then, your main script could search a directory for thumbnail images and loop over each, building up the main page dynamically by assembling all the little blocks of html generated by the template.