I don't work with DB files, so I tried something similar. I passed a CGI param and then did ++$HASH{ $p->param('box')} and it worked. That was even with using string values inside the hash. So, again, pardon my ignorance with DB files, but can you maybe not change the values in the way that you are trying because of the 0644 permissions. That is rw-r--r, and maybe the Perl script is not running as you... Just an idea,
Chris