Well, as I said, you cannot call an object method if the object does not exist and its values aren't set, so simply try this instead :-)

my $cgi = new CGI;

# get the User
my $user = new GWAK::User();

# did it work ?
if (!$user) {
  # if not go to login page
  print $cgi->redirect("insert your login_url");
} else {
  # or proceed
  print $cgi->header();
  ...
}
[download]

caedes is right, since you once write a header you cannot write a second one, but in my experience a mistakenly written second header would then just occur as plain text, but that might differ depending on the default mime-type setting of the webserver.

Have a nice day
All decision is left to your taste