$re = qr{
  (??{ '\b\w{1,' . ($len - ($+[0] - $-[0])) . '}\b' })
  (?(?{ ($+[0] - $-[0]) < $len })
    (??{ '\W{0,' . ($len - ($+[0] - $-[0])) . '}' })
  )
}x;

s{
  (
    (?:
      (?(?{ ($+[0] - $-[0]) == $len })(?!))
      $re
    )+
  )
}{$1\n}xg;