This is some very nice work. A few notes:
- You don't refer to the package name consistently in the docs. Or rather, you do, but you consistently refer to it as "gmail" (all lc) rather than "Gmail".
- When formatted via perldoc or similar, the formatting of the same code is a little wacky. Not sure why this is... the code seems to display OK in my browser, but maybe you are using tabs rather than more conventional 4-space indenting?
- When fetching message info, Gmail is apparently escaping the < and > characters. You might want to look at those strings when you get the data back and clean them up a bit, just as an ease to users.
Again, nice work!