AFAI Understand your pb, I may schematize it like:

     +----------------+
     | Client Browser |
     +----------------+
        |^     ^ |^
        ||    /  ||
    [A] ||   /   || [C*]
        ||  /    ||
        v| /(B2) v|
+----------+   +-----------+
| HTTP Svr |   | HTTPS Svr |
+----------+   +-----------+
     (B1) \     ^ (C1)
           v _ /
            / \
           |\_/|
           |   |
            \_/
[download]

Where:

• A: POST/OK action(request)/responses(status) exchanges required to validate form datas.
• B: (1) storage of form datas and (2) HTTP 30(2|3) response reply when data are validated to redirect client to secured server (without forgetting passing a kind of index to stored datas).
• C: GET/POST/OK action(request)/responses(status) exchanges around secure datas. GET (required, see below) would retrieve previously stored datas.

You may report to this (Redirection of a POST transaction to another URL as POST) for more details about requirements.

Caution: I've not tested this stuff, but you could merely post your perl solution to CUFP ;)