:) See perlre:

(??{ code })

WARNING: This extended regular expression feature is considered highly experimental, and may be changed or deleted without notice. A simplified version of the syntax may be introduced for commonly used idioms.

This is a "postponed" regular subexpression. The code is evaluated at run time, at the moment this subexpression may match. The result 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 where 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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I just noticed I made an assumption too many myself - code is updated.

Makeshifts last the longest.