Set up your session before you print out the header. printing it twice doesn't work.

also, doing

my $q = CGI->new;
my $session = CGI::Session->new("driver:File", $q, {Directory=>’/tmp’}
+);
print $session->header();
[download]

is a lot easier than what you're doing here.

Update: and put the -wT options back at the end of the #! line and did you read this already?