class Object
  unless instance_methods.include? 'instance_exec'
    def instance_exec *a, &b
      m = "__instance_exec_#{ Thread.current.object_id.abs }__"
      singleton_class{ define_method m, &b  }
      send m, *a
    ensure
      singleton_class{ undef_method m  } rescue nil
    end
  end
  unless instance_methods.include? 'singleton_class'
    def singleton_class &b
      sc = 
        class << self; self; end
      sc.module_eval &b if b
      sc
    end
  end
end

a = []
a.instance_exec 42 do |elem|
  push elem
end
p a