There is no "Law of Perspective". It is a term used for an illusion that occurs as we see parallel lines moving away, over the horizon and then seems to meet at a certain point.

What we do know, and can measure to extreme accuracy, is how big an object will appear to be at any given distance. Take for example, a car heading towards you as you stand on a highway over pass. As it travels towards you, it seems to grow larger in size. And if you walk to the other side of the overpass and watch the same car move away from you, it seems to grow smaller. We have equations that can predict what size that car will appear to be at any given distance. And they are completely accurate with reality.

Now, if we exchange the car with the sun, and exchange the highway over pass with the flat earth map, there is a huge problem. If the earth is as big as the flat earther's say it is, and the sun is close to us (3000 miles above us at noon is what I often hear from flatties), then the sun's apparent size should get bigger as it travels toward us in the morning, just as the car on the highway does. Then after noon, the sun should appear to get smaller as it travels away from us, just as the car does on the highway.

Using the formulas that exactly predicts the apparent size of every other object that we have ever measured and using the dimensions of the flat earth model, the sun should grow three times in apparent size as it travels towards us in the morning. And then shrink three times smaller as it travels away from us in the afternoon.

This does not happen. The sun stays the same apparent size all day long. The sun stays the same apparent size because the distance to the sun doesn't change. All that is happening is that the earth is rotating on its axis while maintaining the same distance from the sun.