my @known_mac = ("foo","bar");
my @last_scan_mac = ("foo","karl","heinz","bar");
# Hier sind also nur karl und heinz neu
    for my $a (@last_scan_mac) {
        for my $b (@known_mac) {
            if ($a =~ $b) {
               
 print "$a ist bekannt";
            }
        }
    }