You don't want to use the multiple cgi thing.. It really doesn't serve a purpose in this case.
Ok, so to get this straight, you are saving parameters across sessions and need to re-access those paramaters, keeping intact any new paramaters that come along?
Looking at save() from CGI, it basically writes a file of key=value pairs, one per line.. also, throwing other info at param() will add those key=value pairs to the list.
With that info, this might help:

#!/usr/bin/perl -Tw
use CGI;
my $cgi = new CGI;

# show current list 
print "\n\n";
print join(" ",$cgi->Vars());
print "\n";
 
if (open(FILE, "<./saved_params.txt")) {
  # add saved parameters
  while(<FILE>) {
    chomp;
    # CGI's save() ends the file 
    #  with = alone on the last line    
    last if $_ =~ /^=$/; 

    # split out the key=value pair
    my($key, $val) = split(/=/,$_,2);

    # and add them to the current list
    $cgi->param($key=>$val);
  }
}
 
# show final list
print "--\n";
print join(" ",$cgi->Vars());
print "\n\n";
 
# save final list
if (open(FILE, ">./params")) {
  $cgi->save(FILE);
  close(FILE);
}
[download]

I ran this from the cmd line, so it won't work as is in a web environment (no print $cgi->header in there). Also, it doesn't do any checking for a parameter that's already existing, so if the original list had like foo=bar and there was a saved foo=baz, the baz would overwrite bar (i would imagine), but that should be easy enough to check against.
Hopefully that'll help you with what you're trying to do.
-Syn0