What is view confusion in perspective projection?

I read the definition of view confusion that if any object exist behind the COP (center of projection), then it can be projected onto the view-plane seems like upside down and backward.

My question is that any object which is behind the camera how this object projected to view plane which is in front of camera?

The basic mathematics of a perspective projection have to do with lines through the center of projection. A line that passes through the center of projection and that is not parallel to the view plane will intersect the view plane at a single point. With the exception of the center of projection, every point on that line is projected to the same point in the view plane. That includes points far beyond the view plane in front of the camera, points behind the camera, and points between the camera and the view plane.

Remember that in computer graphics there is no actual camera involved and hence it is possible to get strange effects like this where the view screen can "see" points in both directions from the camera ("in front" and "behind"). In real life a typical camera can get pictures only from one general direction, not two opposite directions, but this is not real life.

In fact, the usual representation of the view plane in computer graphics, with the center of projection on one side of the view plane and objects in a scene on the other side of the view plane, opposite from the camera, never occurs in a real camera; there is no camera that can capture an image before the light has passed through the focal point of the camera lens. The way a real-life camera works is that the center of projection (aka focal point) and the scenery are all on one side of the view plane (the negative in chemical photography or the sensor in digital photography) and the lens projects the image of the scenery upside-down and backward on the view plane. We get a right-side-up picture by turning the image $180$ degrees before showing it to people.

Since you tagged projective geometry, I feel obliged to point out that in at least some mathematical applications of projective geometry, this so-called "view confusion" is a feature, not a bug. So is the so-called "topological distortion" described here or here.

When someone points out "view confusion" or "topological distortion" to you, it is merely a reminder that if you want "realistic" computer graphics you need to pay attention not only to the lines of projection; you also need to make sure you are only looking at parts of the scene that are "in front of" the camera.