gistfile1.rbx

 % cat evaltest.rb

class Zoo
  def bar=(a)
    puts a.inspect
  end
end

class Foo
  def initialize
    @baz = Zoo.new
  end

  class_eval <<-EOS, __FILE__, __LINE__
  def bar=(*args, &block)
    @baz.bar=(*args, &block)
  end
  EOS
end

Foo.new.bar= 1