$number_set = "6 7 8 9 10";

##</code><code>##

$number_set = "6 7 8 9 10";
$number_set =~ s/ (?>(\d+))
                     # prevent backtracking, so that
                     # a failed match after a '10'
                     # won't try 1 and 0 individually
                  (?{local $first = $1})
                     # set $first eq to the number
                     # just matched
                  (   
                     \s
                        # match a space
                     ((??{++$first}))
                        # followed by the number after
                        # the one just matched
                  )+
                     # as many times as possible,
                     # but at least once
              /$1-$+/gx; 
                 # and replace it with the original $1,
                 # a dash, and the last number matched