Actually your regular expressions are:

$exp1 = "123*";
$exp2 = "123+";
[download]

* is "any number of" the preceding character, including zero.
+ is "one or more of" the preceding character.

Hence: /x+/ == /xx*/

Apart from the pedantry, I'm afraid don't have an answer for your question.

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no flames if this changed for 5.10, because that's the Perl I live in.