I'm not sure what you're trying to do here. Windows already does DHCP for you. But nevertheless:

1. use strict;
   use warnings;
   [download]

2. When I run this on Strawberry 5.18.1 MSWin32-x64-multi-thread on Windows 7 x64, I get:
   "Can't create socket: IO::Socket::INET: Only one usage of each socket address (protocol/network address/port) is normally permitted."

This makes sense as my local UDP 68 port is in use by svchost.exe for DHCP services already. When I switch to a non-well-known-port (e.g., 6800) it runs but it doesn't send the packet out the interface I expected - which may be your problem when using the alternate NIC. I determined this with a packet capture (Wireshark) capturing on all interfaces with filter 'udp.srcport == 6800'.

Windows has a preferred interface to send traffic out when there are several matches in the host route table. In this case, your destination of 255.255.255.255 could be for *any* network interface / card, so the default one is used, which by priority, may not be the one you "want".