The funny thing happening here is that your web server runs under a different user, usually apache|nobody|www. Probably the directory you are trying to write into is owned by you and not by the user that runs httpd. So, if you check $! you will probably see a permission denied error.
The solution is to change the owner of the directory where your script will write its files. Now you need to discover who should be the owner :)

Ciao, Valerio