class_self.rb

#!/usr/bin/env ruby

class Hoge
  def foo
    a = 123
    (class << self; self; end).class_eval do
      define_method(:bar) do
        puts a
      end
    end
  end

  def baz
    a = true
    singleton_class.class_eval do
      define_method(:qux) do
        puts a
      end
    end
  end
end

h = Hoge.new
h.foo
h.bar # => 123
h.baz
h.qux # => true