Split Eye SM64 is a hack made by rovertronic that alters the camera functions in vanilla Super Mario 64, and nothing else.
Use D-Pad left & right to split the camera. You can also hold L to speed it up.
As the image used for this hack implies, Mario has gained the ability to move his eyes independently, albiet only on the x axis. Each eye can be adjusted using the D-pad, and the further they are adjusted, the stranger and more disorienting Mario's view becomes.