sub login {
      $/ = "\015\012";
      print $client "username:";
      $login=<$client>;
      chomp $login;

      if ($login eq "") {
        print $client "Must enter a username!\n\r";
        &login;
      }

      $/ = "\012";
      open (FILE1, "users.txt") || print "Error occured while opening file";
      while (<FILE1>) {
        ($first,$second ) = split /:/;
        print $first;
        if ($login eq $first) {
          print $client "Sorry this username has been taken, try again!\n\r";
          &login;
        }
        $/ = "\015\012";
      }

      close (FILE1);
      ...