# Is planarity really necessary for conjugation?

If we look at the $\ce{ClO4-}$ anion, the chlorine atom in it is sp3 hybridised. Hence, it has a tetrahedral structure in which conjugation shouldn't be possible, but while explaining the acidic strength of $\ce{HClO4}$, I've seen many people claim that it is very acidic because the $\ce{ClO4-}$ anion is stable due to the conjugation of charge between the four oxygen atoms.

So, all of this boils down to: Is planarity really necessary for conjugation?