From language/subscripts

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multi method ASSIGN-KEY (::?CLASS:D: $key, $new)

Expected to set the element associated with $key to the value $new. Implementing this is entirely optional; if you don't, self.AT-KEY($key) = $new is used instead, and if you do, you should make sure it has the same effect.

This is meant as an opt-in performance optimization, so that simple assignments %age<Claire> = 29 can operate without having to call AT-KEY (which would have to create and return a potentially expensive container object).

Note that implementing ASSIGN-KEY does not relieve you from making AT-KEY an rw method though, because less trivial assignments/modifications such as %age<Claire>++ will still use AT-KEY.