gistfile1.txt

class Foo does Associative {
    has %!elems;
    method at_key($k) {
        Proxy.new(FETCH => { %!elems.exists($k) ?? %!elems{$k} !! 0 },
                  STORE => -> $, $val{ %!elems{$k} = $val } 
        );
    }

}

my $a = Foo.new;

$a<foo> = 2;
$a<bar> = 3;

say $a<foo> - 1;