sub enter_number {
  local $_;
  while () {
    print "Enter new room number:  ";
    chomp ($_ = <STDIN>);
    if ($_ =~ /^4\d\d\d$/) {
      return $_;
    } else {
      print "\nMust be a 4-digit number beginning with '4'; try again.\n";
      next;
    }
  }
}