/([$set]).*\1/ # Look if there is any repetition
  or 
  /[^$set]/      # Look if there is any unallowed character

# Which combines to

  /([$set]).*\1|[^$set]/

##</code><code>##

my $set = "ABCDEF";
for (<DATA>) {
  chomp;
  print "$_ vs. $set : ", $_ =~ m!([$set]).*\1|[^$set]! ? "Yes" : "No", "\n";
};
__DATA__
A
B
C
ACX
ABC
AA
BB

##</code><code>##

perl -MHTTP::Daemon -MHTTP::Response -MLWP::Simple -e ' ;    # The  
$d = new HTTP::Daemon and fork and getprint $d->url and exit;#spider
($c = $d->accept())->get_request(); $c->send_response( new   #in the
HTTP::Response(200,$_,$_,qq(Just another Perl hacker\n))); ' #  web