This may or may not help, depending on the nature of your project.

There is a company that sells software to convert (among other things) WordStar to HTML (or XML). Here's a link:

stellent.com

I have no idea how much it costs, but based on the 50K files I'm going to assume you might be doing this for a company that can afford to pay for software, or a nonprofit that might get it for free.

There is also a free utility for DOS, with cool language features, and it might also do the job (I'd try this first):

wordstar.org

Otherwise I'd guess it might be possible to wrap a script around OpenOffice or similar.

Sorry, I really wanted to say something like use Doc::WordStar qw(convert_to_html convert_to_ascii);