For sure someone else will answer you about the cookies, but, please tell me:

1. Wy do you use local here? I think you want to use lexical variables here and those are declared with the my keyword.
2. Why did you prefix your subroutine calls with the & sigil? Are these subroutines prototyped and you want to avoid the prototype checks? write_cookies isn't prototyped so you should drop the &. It serves no useful purpose.

Also, I try not to have variables inside my subroutines that are not passed in as parameters, unless it are really and truly global variables:

sub write_cookies{
     my ($username, $password, $regtype, $expires) = @_;
     print "Set-Cookie: reguser=$username; expires=$expires; path=/;\n
+";
     print "Set-Cookie: regpass=$password; expires=$expires; path=/;\n
+";
     print "Set-Cookie: regtype=$regtype; expires=$expires; path=/;\n"
+;
 }
[download]

And then call the sub as follows:

write_cookies($username, $password, $regtype, calc_cookie_expires(-1))
+;
[download]