If readline encounters an operating system error, $! will be
set with the corresponding error message.  It can be helpful to
check $! when you are reading from filehandles you don't trust,
such as a tty or a socket.  The following example uses the
operator form of "readline", and takes the necessary steps to
ensure that "readline" was successful.

     for (;;) {
         undef $!;
         unless (defined( $line = <> )) {
             die $! if $!;
             last; # reached EOF
         }
         # ...
     }