Unmatched ( before HERE mark in regex m/begin                # Start with 'begin',
          ( << HERE ?:                  # Followed by
             (?>[^be]+)        #    Not a b or e, many times,
            |b(?!egin)         #    Or a b not starting 'begin',
            |e(?!nd)           #    Or an e not starting 'end',
            |(??{ $re })       #    Or a balanced begin/ at fudge line 6.