Note: I only checked amd64 arch. Mechanize requires >dev-perl/libwww-perl-6 and dev-perl/HTML-Form OR <dev-perl/libwww-perl-6. Since libwww-perl-6 is now stable, the first dep should be selected by portage, however, since dev-perl/HTML-Form is not stable, it fails. The solution is pretty simple: stabilize dev-perl/HTML-Form. Reproducible: Always
HTML-Form should have been stabilized as part of bug #386309. Fixed for amd64. @hppa,arm: Your arches probably don't strictly need a stable HTML-Form. But it's a form of a regression from the stable libwww-perl-5 with HTML-Form to libwww-perl-6 with no stable HTML-Form. HTML::Form is used by: net-mail/spamcup dev-perl/WWW-Mechanize-FormFiller dev-perl/WWW-Mechanize
Stable for HPPA.
Fixed, both >=dev-perl/libwww-perl-6 and dev-perl/HTML-Form are stable for all arches.