I was at Google for a long time and it was almost entirely Perforce (some Googlers used git) until it was Piper (which is very similar to Perforce). L&S used CVS at Stanford for the original Backrub/Google code and for some time after the establishment of Google, Inc.
It was built from scratch because it was designed as a replacement for Perforce because Perforce is expensive and the G used wrappers around it, anyways.
The commands are still basically the same as Google's wrappers around Perforce and the learning curve was non-existent for someone who was used to using Perforce at Google.
