refinements.rb

class A
  def say
    puts "A"
  end
end

module B
  def self.test
    ref = refine(A) { }

    block = Proc.new do
      def hey
        puts "b"
      end

      def say
        hey
      end
    end

    ref.module_eval(&block)
  end

  test
end

class C
  using B

  def say
    A.new.say
  end
end

A.new.say # => A
C.new.say # undefined local variable or method `hey' for #<A:0x007f9e4c0a9cf8> (NameError)