If there are multiple conditions (IPv4 may or may not be there, IPv6 may or may not be there), then there should be multiple conditionals. So rather than testing if the key device is defined, you should test if ip and if ip6 are defined, like:

for my $nic (networkInfo()) {
  if (defined $nic->{ip}) {
    print "DeviceMain: $nic->{device} has IP Address $nic->{ip}\n" .
        "\tMask: $nic->{mask}\n" .
        "\tBroadcast: $nic->{bcast}\n";

  }
  if (defined $nic->{ip6}) {
    print "DeviceMain: $nic->{device} has IPv6 address $nic->{ip6}\n";
  }
}
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Note I also added defined tests. This has no real impact here (unless these values can be meaningfully defined to empty strings or the value 0), but it is probably a good habit to get into since later it will avoid unwanted autovivification side effects for complex structures as well as weird bugs when those values are legal.