sub try_again {
    my ($r,$msg) = @_;
    $r->pnotes(user_form_err => $msg);
    my $subr = $r->lookup_uri('/register.thtml');
    $r->filename($subr->filename); 
    return Apache::Template::handler($r);
}
sub my_handler {
    ...
    something() or return try_again($r, $msg);
    ...
}