Calm down, Class::DBI is much easier than lots of other things you've already learned.

I'm guessing you are getting into trouble because you are calling set_up_table() before you call set_db(). That won't work because set_up_table() needs to talk to the database to find out about the table. So, load your other modules after you call set_db(). You could use require(), or you could put the call to set_db() in a BEGIN {} before you use() the other modules. And don't put them all in one file like that. It's cool, but not a good idea in the long run.