Can't I put the "config" call in other place so that its effect is already there when I call load_tmpl() in the first run mode?

Make your own method that calls load_tmpl with the defaults your want

You don't even have to use hook mechanism

sub lt {
    shift()->load_tmpl( @_, utf8 => 1 );
}
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Or use the CGI::Application::Plugin::AnyTemplate plugin as it supports configuration of template drivers