Perfect pitch - not so perfect?
Here a BBC website article about a study that reveals that "perfect pitch" is not quite as perfect as has been thought, and can change according to circumstances.
Speaking for myself, I can usually detect whether the A string of my violin has gone slightly flat, without reference to another source, but this is not unusual for musicians who are used to playing in orchestras or other ensembles and are exposed regularly to a tuning "A" at 440Hz. I’m sure that is no more than a straightforward memory effect.
What I certainly don’t have is the kind of perfect pitch (which some unfortunate individuals apparently have) that makes it difficult for them to play or listen to music tuned to other than A440; which is why I have no problems in playing my piano which is tuned to A430 (= C256).