Defining attr_accessor for class instance variables - Ruby

Question

I am trying to create an accessor for a class instance variable. I am calling the attr_accessor method from a module which is included in the class. See the code below:

module Persistence
  def self.included(mod)
      mod.extend ClassMethods
      # Add accessor for class instance variable
      class << mod
          attr_accessor :persistent_data
      end
  end

  module ClassMethods
      def X
          persistent_data = 'data'
      end
  end
end

The above code works. However when I change the code which calls attr_accessor, to this:

 mod.instance_eval do
     attr_accessor :persistent_data
 end

I get NoMethodError: undefined method `persistent_data='

Shouldn't both ways work the same or is my understanding wrong here? I am using REE 1.8.7