And buyers buy -- demand -- and sellers sell -- supply -- according to their perceptions of value.
Often they don't. Someone can buy a stock they perceived to be overvalued (or sell an undervalued stock), for all kinds of reasons - they think the price will go higher; it hit their stop loss; they want to alter their risk profile; they are hedging another position; their fund is suffering redemptions and they have to cover; the government made it illegal to continue holding short positions; they are trying to take over the company for strategic reasons; they hit the wrong key on their computer terminal; their algo went haywire; they felt like gambling on it.
None of these have anything to do with their perceptions of value. Some of them don't have to do with their perceptions at all (e.g. forced buying/selling, such as redemptions, stop-losses etc).
Remember I am arguing against the statement "perception is reality". I am definitely NOT arguing against the statement that people often act on their perceptions. The latter statement is obvious, the former is absurd. Also, people don't always act on their perceptions. When a person's head is cut off by a sword, and they fall dead to the ground, their actions are governed by objective reality, not by their perceptions. Similarly, there are reality-driven actions in the markets as well, it is not all about perception.