# initialization
# the hash describes regexps, requiredness, and other checks
# per input key
my $validator = My::Class->new ( { ... } );

# later when processing request
my $form = $validator->validate( { get => "params" } );
if ($form->is_valid) {
    do_something( $form->data );
    redirect( "/somewhere" );
} else {
    show_form_again( display_errors => $form->error, input_defaults => $form->raw );
};


##</code><code>##

if (!load_user($form->data->{user})) {
    $form->error( user => "No such user in database" );
};