use List::AllUtils qw/reduce/;
my @list = sort {$a <=> $b} (1, 2, 3, 6, 9, 10, 13, 22, 20, 19, 15, 21);
print reduce {
    if(my ($sign,$last) = $a =~ /([,-]?)(\d+)$/){
        if( $last+1 == $b ) {
            $a =~ s/[,-]?\d+$// if($sign eq '-');
            $a.= "-$b";
        } else {
            $a.=",$b";
        };
    };
    $a;
}@list;