As pointed out already, eyes (and brains) don't process all at the same speed, so there is no magical perfect FPS number for all humans. People also claim to see a difference all the way up to 120 MHz, which is only kind of true.

To explain as simply as I can, if you're not processing the images at the same exact frequency as they are being presented, then you will notice disparity. If you process images at 30 FPS, and they are generated at 34 FPS, then you're going to be processing more transitional images (one image morphing into the next) than clean images. Your eye is taking a "snapshot" of what you see more often at times when the image is in the process of changing to the next one, because you're not perfectly synched up with the rate they are delivered.

Higher FPS means the transitions from one image to the next happen much faster, and less likely to be caught by the eye. So, in theory, there is no FPS threshold that can be claimed to be perfect. But there's theoretical thresholds where you'll stop seeing any difference at all (which will be somewhere in the hundreds if not thousands of MHz).