What you should keep in mind depends a great deal on how much the translator is able to translate current idioms into Perl 6 idioms. On your dynamically scoped $_ issue, it's not clear whether the translator will be smart enough to see that you've invoked length in a context where the $_ will certainly be undefined. If it is that smart, it could certainly install an argument of the form:

    sub len ($_ = $CALLER::_) { length }
[download]

which has much the same effect as if $_ were dynamically scoped.