I don't know what's the state of assertions in 5.10, if they are not implemented I'd like to see some kind of assertion mechanism in 5.12.

You should have the possibilty to disable assertions via a pragmatic use or a command line switch with virtually no performance impact.

Usefull assertions are imho plain assert $code_that_evals_to_true; and pre- and postconditions for subs and methods.

In the case of methods it would be usefull if the pre- and postconditions were inherited to all child classes (see PRE/POST in S06 for the corresponding p6 spec).