Don't know if this solution is really preferable to any of the others, but its got a  s/// in there! Uses state feature (available with 5.10+), but could easily avoid it. Note that it does not bother to range-ize a degenerate range: '1,2' does not become '1-2'.

>perl -wMstrict -le
"use feature 'state';
 ;;
 my $s = '2,3,5,6,7,9,11,12,13,14,16,17,19';
 print qq{'$s' \n};
 ;;
 my $sep = qr{ \s* , \s* }xms;
 my $n   = qr{    \d+    }xms;
 ;;
 sub order {
   state $p = 0;
   my $t = $p;
   $p = $_[0];
   return $_[0] - $t == 1;
   }
 ;;
 use re 'eval';
 $s =~ s{
   (?<! \d)   ($n)   (?{   order($^N) })
   (?=   $sep ($n) (?(?{ ! order($^N) }) (*F))
     (?: $sep ($n) (?(?{ ! order($^N) }) (*F)))+
     )
   .+? \3 (?! \d)
   }
   {$1-$3}xmsg;
 ;;
 print qq{'$s' \n};
"
'2,3,5,6,7,9,11,12,13,14,16,17,19'

'2,3,5-7,9,11-14,16,17,19'
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Update: Actually, the non-capturing look-ahead folderol is not needed. The following  s/// seems to work just as well.

$s =~ s{
  (?<! \d) ($n)   (?{   order($^N) })
  (?: $sep ($n) (?(?{ ! order($^N) }) (*F))){2,}
  }
  {$1-$2}xmsg;
[download]

Update: Even slightly simpler and no numbered capture group variables, but  \K only available with 5.10+, like state.

sub order {
  state $p = 0;
  my $t = $p;
  $p = $^N;
  return $^N - $t == 1;
  }

$s =~ s{
  (?<! \d) ($n) \K (?{   order() })
  (?: $sep ($n)  (?(?{ ! order() }) (*F))){2,}
  }
  {-$^N}xmsg;
[download]