class_eval_test.rb

class Foo
  A = "Foo::A"
  @@b = "Foo::@@b"
end

module MyClassEval
  A = "MyClassEval::A"
  @@b = "MyClassEval::@@b"
  def self.my_class_eval(klass, &block)
    klass.class_eval(&block)
  end

  def self.my_class_exec(klass, &block)
    klass.class_exec(&block)
  end
end

module Bar
  A = "Bar::A"
  @@b = "Bar::@@b"
  Foo.class_eval do
    p [A, @@b, Module.nesting]
    C = 1
  end
  MyClassEval.my_class_eval(Foo) do
    p [A, @@b, Module.nesting]
  end
  Foo.class_exec do
    p [A, @@b, Module.nesting]
  end
  MyClassEval.my_class_exec(Foo) do
    p [A, @@b, Module.nesting]
  end
end