From language/subscripts

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

Expected to bind the value or container new to the slot associated with $key, replacing any container that would be naturally found there. This is what is called when you write:

my $x = 10;
%age<Claire> := $x;

The generic Hash class supports this in order to allow building complex linked data structures, but for more domain-specific types it may not make sense, so don't feel compelled to implement it. If you don't, users will get an appropriate error message when they try to bind to an associative slot of an object of this type.