We perceive perfect circles just fine, even if the circle in front of us is slightly less than perfect.

For one thing, the cones and rods (the light sensitive elements) on our retina are not laid out in a perfect grid like that you see on a digital cameras CCD sensor.

For another, we don't actually "see" pixels in that sense. The "signal" has been processed to some degree before it reaches your conscious mind. You perceive lines, curves, areas, etc. When you see a circle, you see "a circle". You don't see 3,423 pixels laid out in a pattern approximating a circle.

As for autocad, render the circle at a higher resolution, or use anti-aliasing.