I can't recommend any module in particular -- a quick look on CPAN shows HTML::Strip as the first module I found that might do the job.

While it's tempting to say, "Hey, I can do that with a regular expression!", you'll probably find that it rarely works as well as you expect.

HTML E-Mail sometimes contains the HTML version of the message in one section, and the text only version in another section -- you might want to check that out as well.