To convert a Perforce depot into a mercurial repository, there are two possibilities: The convert-Extension of mercurial or the Mercurial plugin Perfarce.
Both solutions aren’t satisfying because the branches and tags management doesn’t work how it should.
Because of this, I started to create an own converter, named p42hg, which will be available as open source soon. It already performs quite well, under Linux as well as under Windows.