First experiments with

objhash.pl

class ObjHash {
    has %!keys handles (keys => 'values');
    has %!values handles <values elems>;
    method at_key(Mu $key) is rw {
        my $lkey = $key.WHICH;
        %!values.exists($lkey)
            ?? %!values{$lkey}
            !! pir::setattribute__0PPsP(my $v, Scalar, '$!whence',
                    -> {
                        say "in whence; v='$v'";
                        %!keys{$lkey}   = $key;
                        %!values{$lkey} = $v ;
                    }
                );
    }
    method bind_key(Mu $key, \$value) {
        my $lkey        = $key.WHICH;
        %!keys{$lkey}   = $key;
        %!values{$lkey} := $value;
    }
    method pairs() {
        %!keys.keys.map({; %!keys{$_} => %!values{$_}});
    }
    method kv() {
        %!keys.keys.map({; %!keys{$_},   %!values{$_}});
    }
}

# works:
my $x = ObjHash.new;
my $a = (class A { }).new;
$x{$a} := 'foo';
say $x{$a};
say $x.keys.[0] === $a;

# fails:
$x{'foo'} = 'blub';