if ($domain =~ /[A-Z0-9]+[A-Z0-9-]+[A-Z0-9]/i) {
  # do A
} else {
  # do B
}

##</code><code>##

# /[A-Z0-9]+[A-Z0-9-]+[A-Z0-9]/i matches
someDomain
someDomain..
..someDomain..
_!@#!@#!@#01a!@#$
# and so on

##</code><code>##

if ($domain =~ /([A-Z0-9]+[A-Z0-9-]+[A-Z0-9])/i) {
  # $1 contains what is in the first pair of parentheses
  print "$1 matched\n\n"; 
} else {
  print "$domain did not match\n\n";
}