I'm wondering if there's a way to do this with WMI. On my Win2003, I can:
C:\chas_sandbox>wmic nic list /?
and it looks like MACAddress and PermanentAddress are values you can get on the "full" listing. When I do it, though, all the PermanentAddress-es are empty, and I'm not sure how you'd distinguish that you had the NIC you wanted (the one on the motherboard).

In Re^3: killing on win32 I found Win32::Process::Info::WMI, but I haven't found one related to NICs. Perhaps DBD::WMI? but then you'd have to craft your own WMI query.