You'll find that many modules have documentation inline with the code. For example, Text::CSV code is in "C:\Perl\site\lib\Text\CSV.pm" (base path depending on where you installed Perl, of course, and file path may be slightly different if you didn't use ppm to install the module). But that file also contains all of the documentation you see when you invoke perldoc Text::CSV. I have Text::CSV 1.10 installed, and the documentation is all at the end of the .pm file (beginning around line 220 or so).
Some modules also have HTML-ified documentation. Text::CSV is one of those. This file is at "C:\Perl\html\site\lib\Text\CSV.html". I'm not positive, but I think that's due to installation through later versions of ppm. I have many more modules installed (in C:\Perl\site\lib) but very few other html files.
But I've never found a module that, once installed, didn't have any documentation via perldoc Module::Name. So I tend to use that a lot more than looking for any HTML files.
Added: Not sure why this is being downvoted. If you feel the need to downvote and have a chance to reply or /msg, I'd appreciate hearing your reasoning.