Aha, so we're talking about a specific construct, the http://perldoc.perl.org/perlre.html#%28?{-code-}%29, which says
inside a (?{...}) block, $_ refers to the string the regular expression is matching against. You can also use pos() to know what is the current position of matching within this string.

In reply to Re^2: Value of $_ inside a regex (?{ code }) by Anonymous Monk
in thread Value of $_ inside a regex by Athanasius

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