If I understood correctly, you want to read VendorTxCode from inside finalorderpage.shtml, probably using some kind of 'server side include' script. In that case I would do something like:

# inside finalorderpage.shtml
<!--#include virtual="/cgi-bin/intercept.cgi">

# intercept.cgi
#!/usr/bin/perl

use CGI;

# scripts executed via SSI don't get usual informations,
# so we must rebuild the query passed to our calling page
# using %ENV (it's a dirty trick)

my $req_uri = $ENV{REQUEST_URI};
my $query_string = substr($req_uri, index($req_uri, '?')+1);
my $q = new CGI($query_string);

print $q->header('text/html');
print "You called me with VendorTxCode=", $q->param('VendorTxCode');
[download]

There are a lot of assumptions here...

HTH, Valerio