I think the error message is misleading - you are setting up a DB handle but not the one you later request:

      $self->dbh_config('subscriber', 
                [$data_source,
                $self->config_param('mysql.username'),
                $self->config_param('mysql.password'),
                { RaiseError => 1 }
                ]);
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... vs ...

 $self->dbh('subscribers')
[download]

Consider using either a constant or global variable for the name of the database handle (subscriber vs. subscribers).