I use the following, which allows me to use a base class that is defined in a config file - Class::DBI has subclasses for MySQL, pgsql, and others, so I found this to be handy.

use Mayhem::Config qw/get_config/;

my $base_class_name;

BEGIN {
     my $config = get_config();
     $base_class_name = $config->perl_object_base_class;
     eval "use base '$base_class_name'";
     eval sprintf("__PACKAGE__->set_db('Main', %s);", $config->db_conn
+ect_string);
}
}

1;
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where $config->perl_object_base_class returns items of the form 'Class::DBI::SQLite' and $config->db_connect_string returns something like '"DBI:sqlite:dbname", "username", "password"' This allows me to run my system against SQLite or mysql or postgres with just changing the config file (and having the databases setup correctly of course ;)