Perfect pitch works by matching autocorrelation peaks between heard sound and remembered notes

Evidence

Pitch, as we will see, is indeed quantitative in the sense that it is keyed to features in the autocorrelation of sounds. If you have perfect pitch, it is because you can match a signal with a prominent peak in the autocorrelation of the sound at, say, 0.0051 second with a key on the piano with a prominent peak in its autocorrelation at the same time. You will have hit the key G3. — Eric J. Heller, Why You Hear What You Hear- An…, 2019