How about nothing?

given ($foo) {
  when (1) {print "One";};
  when (2) {print "Two";};
  print "Didn't fit any case we know about";
}
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Indent to taste.

UPDATE
I don't think that "normal code" will often see a difference. While I can dream up situations where I think it matters, in all of them you are doing evil, awful things that you probably shouldn't. (Like using a default to shortcircuit out of a function.)