Although you clearly state that we should "assume database, user, pass are defined and correct," that is not what your error message is telling us. DBI->errstr should return the error from the database engine, itself.

Try, from a prompt, the command line client:

$ mysql -u myusername -p
[download]

Your grant statement must grant specific privileges on certain tables to a user @ a domain, identified by 'a password'. If you have root on the db engine, you can use show fields from mysql.tablename and select from mysql.* to see what the mysql installation thinks its valid users are. Do these tokens work at a cli client?

-- Hugh