The third occurence of a string can be found by stepping through your substring with index. Since ',' is a simple string, not a pattern, index is ideal.

On the other hand, if you really do want to search for a particular pattern, here's one way to do it with respect to finding the third comma, using a regexp:

use strict;
use warnings;

my $string = "a,bc,d,efg";

if( $string =~ m/(?:,[^,]*){2}(,)/ ) {
    print $';
}
[download]