I ported Mozilla::Mechanize from Win32::IE::Mechanize, which was in turn ported from WWW::Mechanize. You're right they're not 100% compatible, but mostly they are. I've meant to get a new release out of Mozilla::DOM and Mozilla::Mechanize for a long time, especially since Boris's Mozilla::* modules came out, but I have a little problem where all Gtk2::MozEmbed-based applications segfault for me on Ubuntu, so I'm not able to confirm that things work.
(FWIW, I have seen https://launchpad.net/ubuntu/+source/firefox/+bug/26436
and I have set LD_LIBRARY_PATH and /etc/ld.so.conf and done /sbin/ldconfig, but things still segfault for me.)