I don't think you need a cookie. Instead of printing out a page from the CGI script, re-direct the user to a new page using an HTTP redirect.

open FH, ">>report_sum.txt" 
  or die "Cant open report_sum.txt Reason: $+!"; 
my $writestr="..."
print FH $writestr, "\n";    
close FH;

print redirect("http://mysite.com/thankyou.html");
[download]

NB: Make sure you don't print out anything else (including other HTTP headers) before the redirect.

</ajdelore>