I am a professional musician (bassoon player in a symphony orchestra). I can, if I practice it a couple of times a week, achieve and sustain perfect pitch. I had no semblance of perfect pitch until I decided to practice it at age 24. Before that the only chance I had to guess a note was to put in in relation to my own voice. Even though I don't practice this anymore (it isn't very useful), I still often just instinct…
I took a music theory class in college that had a very heavy ear-training component. The TA in the course would often talk about the concept of "pitch memory", both absolute and relative. In other words, perfect pitch isn't a binary concept of you either have it or you don't, but it's your ability to remember and reproduce absolute pitches from memory. What we think of as "perfect pitch" is the extreme version of thi…
I had an ear training teacher who would play a bunch of random atonal notes on the piano between exercises to "reset" our ears. Only works for relative ears.
Similarly I (relative pitch) retain the key of a piece I practice. But if someone plays a random sequence of pitches it pretty easily makes me lose my anchor or at least make it lose focus by a semitone or three.
Whereas the "real" perfect pitch people are always anchored, no matter how much noise anyone throws at them. Which is also a source of discomfort and difficulty, for example when they need to transpose or work with a different A4 than their "internal" learned one. Where us relative normies would just shrug and accept the new A4.