[問題] eval for string

[sixth (12345(6))]

請問如何讓範例2 像 範例1 一樣，印出計算結果?

範例1:
      eval '$sum = ((0x1 << 8) -1)';
      print "The sum is $sum\n";

      ==> 會印出 255

範例2:
      $str_test = "((0x1 << 8) -1)";

      eval '$sum = $str_test';
      print "The sum is $sum\n";

      ==> 會印出 (0x1 << 8) -1)


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