open STDERR, ">errlog.txt"
  or die "Whoa! I can't even log my errors: $!";

##</code><code>##

open ERRLOG, ">errlog.txt" or die;
$SIG{__WARN__} = sub {
  my ($msg) = @_;
  $msg ||= q{Warning: something's wrong};

  # Add the file and line number to $msg here
  # using the return values from caller().

  print ERRLOG, $msg;
};