From language/operators

See Original text in context

The smartmatch operator aliases the left-hand side to $_, then evaluates the right-hand side and calls .ACCEPTS($_) on it. The semantics are left to the type of the right-hand side operand.

Here is a partial list of some of the built-in smartmatching functionality. For full details, see ACCEPTS documentation for the type on the right-hand side of the operator.