module_variable_test.rb

require 'active_support'

module A
  extend ::ActiveSupport::Concern

  class_methods do
    def sample=value
      @sample = value
    end

    def sample
      @sample
    end
  end
end

class B
  include A
end

class C
  include A
end

B.sample = "ok"
C.sample = "ng"

puts B.sample
puts C.sample

# > ok
# > ng