The Timestamp is in the packet header

This is misleading. As Fletch already pointed out, the IP header does not contain a timestamp. In your code, $hdr contains data added by pcap during a capture and $hdr->{tv_sec} is the time when the packet was captured on the wire. It doesn't really have anything to with the IP header at all. Unless you're dealing with packets dumped through pcap you won't find a timestamp header field.