It is very easy to pass hidden fields, and do any of the other tasks that are common to Web Application Development if you use a Perl framework designed for this, namely, HTML::Mason or HTML::Embperl.

Of these two, to pass hidden data in Embperl is a snap. Your HTML documents become dynamic and to simply take the data from a POST/GET to the current page, the current page simply need do the following:

<FORM>

...

[$ hidden $]
</form>
[download]

and for every key-value pair sent to the current page via POST/GET, an appropriate hidden field is created.

Read more about Mason and Embperl they are APIs offering a simple and powerful interface to all common web tasks.