another posting elsewhere (sorry, haven't got the link) where it was mentioned that "use CGI" causes all sorts of problems with the POST method

That has not been my experience. CGI.pm has caused me a grand total of zero problems with POST requests in the quarter century or so that I have used it. Perhaps if you were to provide an SSCCE to demonstrate, someone here could steer you right.

CGI.pm has its flaws but it certainly beats trying to parse parameters or uploaded files by hand from first principles every time.