In the example you linked to -- you're giving a relative path.

Without knowing how you're calling the program (CGI? cron?), I'd assume it's just an issue with a current working directory that's not the same as your script. (this is quite common for perl modules -- you'll want to work relative to __FILE__, or some other similar trick)

But, without code, the error messages, or more info about the calling context, any help you get is going to be pure speculation.