Perl- eval for string

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)


--