Once you beat Fez (with any number of cubes), you can select "New Game +" from the main menu. In this mode you retain your cubes, maps, keys and previously unlocked regions, but you also gain an additional skill. You will get sunglasses at the start of the New Game + that allow you to view the world in first-person.

Unlockable	How to unlock
First-Person View	Return to the Hexahedron in New Game+.
New Game +	Complete the Game.