Roger's answer is correct. I would go further and say you probably want to construct this regex programmatically.

my $thes530 = qr/thes530-....\.otenet\.gr/;
my $millenium = qr/millennium-....\.ccf\.auth\.gr/;

my $regex = qr/$thes530|$millenium/;

if ($host =~ /$regex/) {
    print "Yay!\n";
}
[download]

I'm sure there's a way to join qr's, but I've never tried it.