package MyApp::Db;
use base Class::DBI;

__PACKAGE__->connection('dbi:mysql:dbname', 'username', 'password');

1;

package  MyApp::Db::User;
use base MyApp::Db;

__PACKAGE__->table('user');
__PACKAGE__->columns(All => qw/id username password/);

...


##</code><code>##

package RunModes::User;
use MyApp::Db::User;

...

sub validate_user {
  my $self = shift;
  my $q = $self->query;
  my $user = MyApp::Db::User->search(
               username => $q->param('username '),
               password => $q->param('password')
             );
  if($user) {
    # laugh
  }
  else {
    # cry
  }
}