Nothing hacked about it. They rewrote it completely, keeping just the interface for compatibility. Perforce scales very well but still has a single server at its core - at some point no matter how much money google threw at that machine (it used to be the beefiest single server they had), it just couldn't keep up.

Yes. Sorry, I was a bit sloppy in my expression.

It started out as vanilla, then hacked up, and then the re-write.

I think Facebook's mercurial was/is also a special edition? Or is everything they do upstreamed?