@yhara こんな感じ

fizzi.rb

module FizzBuzzInteger
  refine Fixnum do
    def to_s
      self % 15 == 0 ? "FizzBuzz" :
      self % 5 == 0 ? "Fizz" :
      self % 3 == 0 ? "Buzz" :
      super
    end
  end
end

class A
  using FizzBuzzInteger

  def foo
    p 100.to_s
    #=> "Fizz"
    p (1..10).map(&:to_s)
    #=> ["1", "2", "3", "4", "5", "6", "7", "8", "9", "10"] # lexical scoping
    p (1..10).map{|x| x.to_s}
    #=> ["1", "2", "Buzz", "4", "Fizz", "Buzz", "7", "8", "Buzz", "Fizz"]
  end
end

A.new.foo