$count = 3;
$str1 = "First string";
$str2 = "Second string";
$str3 = "Third string";
$bytes = pack("C C A* C A* C A*", $count, length($str1),
   $str1, length($str2), $str2, length($str3), $str3);;

print for unpack 'x(c/a*)*', $bytes;;
First string
Second string
Third string