Welcome to the monastery,

CPAN is a great module that makes installing other modules much easier.
CPAN doesn't provide the NetPacket/Ethernet.pm module. You need to either install that module manually, or use the CPAN module to install it.

Since CPAN is a core module, it should already be installed where you're using Perl. To use it to install the NetPacket/Ethernet.pm module it should simply be the following:

os prompt>cpan
cpan> install NetPacket::Ethernet
[download]

Update:
CPANPLUS appears to be a successor of CPAN, though I don't have any experience with it yet.