# An intriguing aspect of this behavior is that it can be used to implement private method calls. 
# If you put your class in a module, you can make use of the file's lexical scope for privacy. 
# First, store an anonymous subroutine in a file-scoped lexical:

    # declare private method
    my $secret_door = sub {
        my $self = shift;
        ...
    };

# Later on in the file, you can use that variable as though it held a method name. The closure 
# will be called directly, without regard to inheritance. As with any other method, the invocant is 
# passed as an extra argument.

    sub knock {
        my $self = shift;
        if ($self->{knocked}++ > 5) {
            $self->$secret_door();
        }
    }