I use this mod_perl2 code snippet to successfully parse out a form's field value submitted via POST method:
use CGI;
my $req = CGI->new($r);
my $field_value = $req->param('form_field');
If you don't use CGI;
as illustrated above, and instead, use the following code:
my $req = Apache2::Request->new($r);
my $field_value = $req->param('form_field');
You'll probably succeed in GET method. However, while getting request via POST method, in my case, I got into infinite loop of some king of 'prefetching filter.c(270) error' and the request will never return.