package Set; use Moo; has '_items' => (is => 'ro', default => sub { { } }); no Moo; sub items { my ($self) = @_; [ keys %{ $self->_items } ]; } sub has { my ($self, $e) = @_; exists $self->_items->{ $e }; } sub add { my ($self, $e) = @_; if ( ! $self->has($e) ) { $self->_items->{ $e } = 1; } } 1;