Looks like you don't care about the values of the String parameters, and want to treat B and C the same, so:

def matcher(l: Foo): String = {
  l match {
    case A() => "A"
    case B(_) | C(_) => "B"
    case _ => "default"
  }
}

If you must, must, must extract the parameter and treat them in the same code block, you could:

def matcher(l: Foo): String = {
  l match {
    case A() => "A"
    case bOrC @ (B(_) | C(_)) => {
      val s = bOrC.asInstanceOf[{def s: String}].s // ugly, ugly
      "B(" + s + ")"
    }
    case _ => "default"
  }
}

Though I feel it would be much cleaner to factor that out into a method:

def doB(s: String) = { "B(" + s + ")" }

def matcher(l: Foo): String = {
  l match {
    case A() => "A"
    case B(s) => doB(s)
    case C(s) => doB(s)
    case _ => "default"
  }
}