use strict;
use warnings;

my ($str, $nth, $p);

$str = "dsf,sdg,hjhgj,gjkh";
$p = -1; $nth = 3;

while ($str =~ /,/g) {
    $p = pos($str) - length($&), last if !--$nth;
}
print substr($str, $p) if $p != -1;