Plesk utilizes Suexec as a CGI wrapper for security. The suexec wrapper prevents the CGI script from having permissions set over 755. Also the script should have owner and group specified by SuexecUserGroup directive for the appropriate domain. In Plesk this user is the FTP user for current domain and the group is always 'psacln'.
You can read more about suexec at: http://httpd.apache.org/docs/suexec.html#model