How can I check if there are any values returned?

The calling code needs to know anyway if it wants to do something the returned values, if anything. Why not make it also indicate whether it wants results or not?

(and with 'fails' I mean that it works correctly, but also produces an warning.)

warnings can be suppressed. If the warn builtin is used under the hood, a local $SIG{__WARN__} = sub { }; should help. If not, I'm sure the DBI documentation has something to say about error and warning reporting.