use File::Tail ();

{
   my $getline;

   if ($opt{'c'}) {
      open my $fh, '<', $MAILLOG
         or die("Unable to open mail log: $!\n");
      $getline = sub { return scalar <$fh>; }
   } else {
      my $fh = File::MyTail->new(name => $MAILLOG, tail => -1);
      $getline = sub { return scalar $fh->read(); }
   }

   while (defined(my $line = $getline->()) {
      ...while loop code here...
   }
}