What version of perl are you using? It works as posted for me running perl 5.6.1 on Redhat linux 8.

From perlre:

(??{ code })
    WARNING: This extended regular expression feature is considered hi
+ghly experimental, and may be changed or deleted without notice. A si
+mplified version of the syntax may be introduced for commonly used id
+ioms. 

    This is a ``postponed'' regular subexpression. The code is evaluat
+ed at run time, at the moment this subexpression may match. The resul
+t of evaluation is considered as a regular expression and matched as 
+if it were inserted instead of this construct.

    The code is not interpolated. As before, the rules to determine wh
+ere the code ends are currently somewhat convoluted.

    The following pattern matches a parenthesized group:

  $re = qr{
             \(
             (?:
                (?> [^()]+ )    # Non-parens without backtracking
              |
                (??{ $re })     # Group with matching parens
             )*
             \)
          }x;
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