You might like:

use strict;
use warnings;

my @list = sort {$a <=> $b} (1, 2, 3, 6, 9, 10, 13, 22, 20, 19, 15, 21
+);
printf "%s\n", join ",", @list;

my @newList;
my $runLen = 0;

for my $elt (@list, '') {
    if (! $runLen) {
        push @newList, $elt;
        $runLen = 1;
        next;
    }

    if ($elt eq '' || $runLen + $newList[-1] != $elt) {
        push @newList, "-", $runLen + $newList[-1] - 1 if $runLen != 1
+;
        push @newList, ',', $elt if $elt ne '';
        $runLen = 1;
    } else {
        ++$runLen
    }
}

print @newList, "\n";
[download]

Prints:

1,2,3,6,9,10,13,15,19,20,21,22
1-3,6,9-10,13,15,19-22
[download]