OK.. this gem right out of the manual.

Focal: The Focal parameter dial sets the camera’s focal length. Smaller numbers give wider angles and will add more stretching and distortion as they decrease. Larger numbers give results like telephoto lenses, and result in less distorted but flatter images.

** Perspective**: The Perspective parameter shifts the current camera’s perspective without changing the camera’s location. This can give the illusion that the camera is farther or nearer to objects in your scene.

Seems to be the same thing.???? 
Spinning the dials causes them to both have the same values regardless of which dial one spins.
Must check to see if there is less barrel distortion when setting 19mm Perspective vs 19mm Focal.