gistfile1.rb

module Mod
  
  def self.included(base)
    base.class_eval do
      puts "Included in #{base}"
      p  ObjectSpace.each_object(Class).sort{|a,b| a.name <=> b.name}.select{|a| a.superclass == Person}.uniq.size
    end
  end
  
end

class Person
end

class Foo < Person
  include Mod
end

class Bar < Person
  include Mod
end

#Output:
#Included in Foo
#1
#Included in Bar
#2