On unix based systems you can use /sbin/ifconfig command to find the active ethernet interface and access it from Net::RawIP, I recently faced this problem.
Vivek
-- In accordance with the prarabdha of each, the One whose function it is to ordain makes each to act. What will not happen will never happen, whatever effort one may put forth. And what will happen will not fail to happen, however much one may seek to prevent it. This is certain. The part of wisdom therefore is to stay quiet.