(1) Accessible: privilege issue
For the web scripts (php or perl or whatever) in Linux:

• for php script
  php interpreter (/usr/bin/php) needs the privilege to access and parse it, so make sure the php script is accessible and readable;
  Normally the directory (which hold php scripts) should have the privilege '775' and the php script file itself is '664'.


• for perl cgi script
  Make sure the perl script is executable: normally the directory (which hold php scripts) should have the privilege '775' and the php script file itself is '775'.
  perl cgi is running by itself: in the first line of the perl cgi, is its shabang:
  #! /usr/bin/perl
  When executing, this command line forks a process to execute itself - the cgi process.

Usually for easy setup and access web application, I use a lot of aliases in httpd.conf, e.g.:
Alias /test "c:...desktopwilliamprojectstest"
This maybe the simplest way to locate a web application in httpd.conf:
http://localhost/test/
This kind of configuration is easy and convenient. However make sure to configure it correctly.
There are 2 level of previlege assignment in httpd.conf:

(A) root dir:
The DOCUMENTROOT default can't be access, in the local machine, we'd better to change the previlege like this:

      <Directory />
      Options Indexes FollowSymLinks Includes ExecCGI
      AllowOverride None
      Order deny,allow
      Allow from all
      </Directory> 

(B) individual dir:
Instead of above settings globally, the following just setup the privilege for '/test':

      <Directory /test>
      Options Indexes FollowSymLinks Includes ExecCGI
      AllowOverride None
      Order deny,allow
      Allow from all
      </Directory>

(3) make the interpreter searchable.

e.g., in Windows, if php.exe is not at the %PATH% env variables, the 403 error occurs. So double check php is available in the %PATH%:

(4) Make sure you have permission to use .htaccess file for Apache web server.
If Apache has overrides disabled. you will bump back with a 403 error.

The above solved my most 403 issues.