class Module
  def singleton_object
    if singleton_class?
      ObjectSpace.each_object(Object).find do |o|
        begin
          (o.singleton_class == self)
        rescue TypeError
          false
        end
      end
    end
  end
end

class Example
  PARENTS = []
  class << PARENTS
    FATHER = :father
    MOTHER = :mother
    singleton_object.push(FATHER, MOTHER)
  end
end

p Example::PARENTS
# [:father, :mother]
p Example.constants
# [:PARENTS]