From @cognominal

According to the test below it seems to be runtime because the error appears after
the BEGIN execution.
The "when possible" proviso is necessary because the provided default parameter may be
run time only.

sub func(@​ok=<a>) { say @​ok.perl}; BEGIN { say 'hi'}; func
hi
Type check failed in binding @​ok; expected Positional but got Str
  in sub func at <unknown file>​:1
  in block <unit> at <unknown file>​:1

$perl6 -v
This is rakudo version 2015.11-167-g235d085 built on MoarVM version 2015.11-19-g623eadf implementing Perl v6.b.