It seems to me you've got the process backwards. I would think your main script should spawn a 'generate results' script, and then should redirect to a 'waiting' page which has a metarefresh to itself. The 'generate results' script should write to a temp file, then when its done, rename the temp file to overwrite what the 'waiting' page is, which will be picked up on the next refresh. However you do it, you should not have both processes writing directly to the user. (Update: Ok, I'm not sure if your scheme would work or not...and maybe mine is just another idea...).

Anyway, Win32::Process::Create is one place where you shouldn't use forward slashes to specify paths, at least not in the path to 'perl.exe' (I get errors doing that anyway); use double backslashes there. And your 'die' message isn't very helpful, you should include '$^E' in the message.