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my $c=0;
s/ \(      (?{++$c})
 | \)      (?{--$c})
 | ([^()]+)
 /$1 if ($c==0)/gex;

s{([()]*)([^()]+)}
 {$c += $1=~y/(// - $1=~y/)//;
  $2 x !$c
 }ge;