Q. Why did my object disappear when I toggled on perspective viewing projection?

A. z-translating an object moves the object along the z-axis, that is, nearer and farther from the camera. If you have parallel viewing projection set, nothing appears to happen when you z-translate the object.

If you z-translate an object enough, making the object come toward the camera, it will move in back of the camera. You will not notice this if you are using parallel viewing projection. However, if you then turn perspective viewing projection on, the object will disappear because, since it is behind the camera, it is not displayed.