use strict;

sub list_to_ranges {
  sort @_;
  my $last = shift;
  my $list = "$last";
  my $span = 1;
  foreach my $next (@_) {
    if( $next == $last + 1 ) {
      $span++;
      next;
    } else {
      $list .= ($span>1?"-$next":",$next");
      $span = 1;
      $last = $next;
    }
  }
  return $list;
}

print list_to_ranges( 2,3,4,8,10,11,12,15 ), "\n";
print list_to_ranges( 2,3,4,8,10,11,12,15 ), "\n";
print list_to_ranges( 8,10,11,12,15 ), "\n";