CGI::Application uses CGI.pm behind the scenes so if you don't find something in the docs look there as well. In particular I think the section on MIXING POST AND URL PARAMETERS is what you need.

Based on that information, you can write a function like this:

# Called before CGI::App dispatches to a runmode
sub cgiapp_prerun {
    my ($self) = @_;
    # $self->mode_param so we don't have to go back and change this if
+ we
    # ever decide to use something other than rm
    if ($self->query->url_param($self->mode_param)) {
        # prerun_mode lets you change CGI::Apps notion of the current 
+runmode
        $self->prerun_mode($self->query->url_param($self->mode_param))
+;
    }
    return;
}
[download]

Actually thats kind of overkill. A simpler solution would be to remove rm=mode2 from the forms action and include a hidden field called 'rm' with value 'mode2' in the form body instead.

Also for debugging look into CGI::Application::Plugin::DebugScreen and CGI::Application::Plugin::DevPopup.