Using a unique filename will ensure that someone won't block access to the form and that data won't bleed together and it's almost as simple as using a semaphore. You don't even need to use cookies, just add the unique id to the path_info and read it from there if you want. (Just make sure CGI.pm is outputting the Form HTML).

As far as untainting the passed sessionid, you could do the following.

path_info() =~ /^(\w+)/;
my $session_id = $1;

unless ($session_id){
       # Session doesn't exist so they are new user.
       
       $session_id = makesessionID();
       # Create file and display first form below
       
}else{
       unless (-e $my_path.$session_id){
          # Something is funny if the session doesn't exist.
          # Probably should die 
       }
       # Read in session data from file.
       
       # Form two (3,4,5 whatever) below.
}
[download]

Just my two cents.

-Lee

"To be civilized is to deny one's nature."