How do I install libpcap?

Forget about libpcap - it's not for windows.
As per the link re Net::Pcap for Win32 posted by Anonymous Monk yesterday:
1) install WinPcap;
2) ppm install http://www.bribes.org/perl/ppm/Net-Pcap.ppd

That's the best you'll currently get. (The bribes repo has Net::Pcap ppm packages for perls 5.6, 5.8, 5.10 and 5.12.)

Cheers,
Rob