test.p6

use v6;

class MyClass {
    has $!hidden;
    has $.public;

    method sayhidden () {
        say $!hidden;
    }

    submethod BUILD(:$hidden, :$public) { }
}

my $class = MyClass.new(hidden => "foo", public => "bar");

$class.sayhidden(); # (Any)
say $class.public;  # (Any)