From language/operators

See Original text in context

    sub infix:<=>(Mu $a is rw, Mu $b)

Called the item assignment operator. It copies the value of the right-hand side into the Scalar container on the left-hand side.

The item assignment operator should be distinguished from the list assignment, which uses the same operator symbol = but has a lower precedence. The context of the left-hand side of the = symbol determines whether it is parsed as item assignment or list assignment. See the section on item and list assignment for a comparative discussion of the two assignment types.