Can someone please explain me this perl code line?

$|=1;
srand(time());
$p = "";
for ($k=0;$k<1300;$k++) {
$p .= ",5-$k";

I want to know esp in regard to this initialization
$p .= ",5-$k";

What does (.) means is p not replaced every-time the for loop is incremented.

Thanks