override inner eval()

newal.rb

module Newal
  def eval(str)
    throw :evaling, [true, Kernel.eval(str)]
  end
end

class << self
  include Newal
end

$result = catch(:evaling) do
  <%= user_code %>
end

class Object
  include Kernel
end

def used_eval?
  $result.is_a?(Array) ? $result.first : false
end

def user_code
  $result.is_a?(Array) ? $result.last : <%= user_code %>
end