I think what he's referring to is the
(??{}) facility. From
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;