Maybe this will help you visualize this. (excuse the bad ASCII art)

   YOU    YOU
    |      |
    V      V
  +-----------+
  | WebServer |
  +-----------+
    |       |
    |       +------+
    V              V
 +------+     +------------+
 | Mojo |---->| Php Script |
 +------+     +------------+

Calling Mojo is the blue path. In Mojo, you are posting to the script, which is the red path, and that's probably not what you want to do. You want the green path.

So, you need Mojo to return to you, and YOU will have to post to the php script. which should be roughly something like this:

use CGI qw/:standard/;
my $url = "http://domain.com/test.php";

print start_form('POST', $url);
print hidden($_, $vars{$_}) for keys %vars;
print submit('Click to continue');
print end_form;
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