# Pastebin 7WtvemrS
class A {
    has $.xx is rw = 0; has %.parameters = {:more, :parameters};
    has &.method-cb is rw = {
        "default method-cb".say;
        .xx = 3;
    };
    method do-test {
        "calling method-cb".say;
        self.&!method-cb;

        $!xx.perl.say;
    }
}

my $a = A.new;

$a.method-cb = {
    .xx = 5 + .parameters<more>;
};

$a.do-test;