I think what you'd do if you've already got an existing perl5 clone is you probably can just add the other person's repo as a remote (git remote add xenu https://github.com/xenu/perl5.git); otherwise just clone their perl5 into a new working copy. Once you've done that you should be able to do something like git checkout xenu/mingw-org and it'll set you up a local tracking branch for that.