More than likely, CGI.pm is already installed on your system. If for some reason it isn't, yes, it would probably make life a little easier for it to be installed, but you can use a private copy, provided you use lib at the top of your script. For example:

#!/usr/bin/perl

use lib "/my/module/directory";
use CGI;
[download]

Perl will then look in your specified directory for the module before looking through the the default directories in @INC.

As far as re-coding if you use CGI.pm goes, the short answer is technically, no, you don't have to re-code everything. Others may disagree with me, but personally, I'd be inclined to just use CGI.pm for the parts in question for the time being. Once you get it working, dig into CGI.pm a bit further and see if there are ways to make your script better and more efficient by taking advantage of more of the features of CGI.pm.

That's my 8**(1/3) cents. Hope that helps.
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Kurt