The fact that you set the permissions to 755 means that anyone can read or execute in that directory, but only if they are running as you can they write. I suspect that the webserver is not running as you.

Another basic sanity thing to check, put the name of the dbm file into a variable and include the contents of the variable in the debugging statement. I have spent too much time in the past trying to track down why X was not being created when in fact it was trying to create Y...