Under mod_cgi it's not easy to redirect POST requests to some other location. With mod_perl you can easily redirect POST requests. All you have to do is read in the content, set the method to GET, populate args() with the content to be forwarded and finally do the redirect:
use Apache::Constants qw(M_GET);
my $r = shift;
my $content = $r->content;
$r->method("GET";
$r->method_number(M_GET);
$r->headers_in->unset("Content-length";
$r->args($content);
$r->internal_redirect_handler("/new/url";

Of course that last line can be any other kind of redirect.