The parent post does not mention Mail::Webmail::Gmail at all, so you are not running the code you are replying to.

Note that the bug reports for Mail::Webmail::Gmail show that it has not been maintained since 2006, which makes it quite unlikely for a web scraper to still work.

If you want to continue with the code you have, you will have to install Mail::Webmail::Gmail. Either use your OSes package manager to install the module or install it through the cpan tool:

cpan Mail::Webmail::Gmail
[download]

Most likely, you will find it much easier to acces Gmail through IMAP.