ruby 2.0

prepend.rb

#!/usr/bin/env ruby

class Hoge
  def fuga
    puts "HOGE#FUGA"
  end
end

module Fuga
  def fuga
    puts "HOGE#FUGA#CHILD"
  end
end

class Hoge
  prepend Fuga
end

Hoge.new.fuga
 #=> "HOGE#FUGA#CHILD"