Different behavior of class_eval between Ruby 1.9.2 and Ruby 1.9.3

uh-oh-scope.rb

# Compare the results running under Ruby 1.9.2 and Ruby 1.9.3
class Foo
  def initialize
    @@value = "Set from Foo initialize"
  end

  def report
    @@value
  end
end

class Bar
  def initialize
    @@value = "Set from Bar initialize"
  end

  def report
    @@value
  end

  def monkey
    Foo.class_eval do
      def set(value)
        @@value = value
      end
    end
  end
end

b = Bar.new
b.monkey
f = Foo.new

puts "Before monkeying:"
puts "Bar's class var: #{b.report}"
puts "Foo's class var: #{f.report}"

f.set("Set through Monkey")

puts "After monkeying:"
puts "Bar's class var: #{b.report}"
puts "Foo's class var: #{f.report}"