Woops, it also converts (11,2,3) to "11-3" and (1,2,30) to "1-30". :)

Here's one way to fix it:

sub num2range {
  local $_ = join ',' => @_;
  s/(?<!\d)(\d+)(?:,((??{$++1}))(?!\d))+/$1-$+/g;
  return $_;
}
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The negative look-behind at the beginning and the negative look-ahead near the end prevent the regex from matching only part of a number, like the second 1 in 11 or the 3 in 30.