in reply to opening filehandles, creating files
Chances are, /home/client/www/data_dir is not the real path that you should be using, or, /home/client/www/data is the path on a machine other than the web server. In either case, the "no such file or directory" is quite clear that the executing script is not able to find one: /home, /home/client, /home/client/www, or /home/client/www/data_dir.
If it was a permissions problem, you would get "permission denied." If it was a tainting problem, you would see a taint error from Perl.