I think winpcap is already installed in my system

Then it's installed in a location that's not being found by default. You need to follow the advice offered by the error message regarding If it is installed in a non-standard location
First, cd to the Net::Pcap build directory and run dmake realclean. Then run, as suggested:

perl Makefile.PL INC=-IC:/WpdPack/Include "LIBS=-IC:/WpdPack/Lib -lwpc
+ap"
[download]

You'll need to modify the INC and LIBS paths specified in that command so that they point to the location of the wpcap headers and library on your machine.
Then run dmake test followed by dmake install

Cheers,
Rob

UPDATE: Give that a try, though I'm now not so sure that's what's needed. I've just noticed this in the message you posted:

socket.h patched... ok
looking for -lwpcap... yes
checking for pcap_lib_version() in -lwpcap... no
[download]

So, at some point, wpcap is being found, but that library doesn't provide a pcap_lib_version(). Is your version of that library a recent one ?