Since this works as expected, I added a test to S04-declarations/implicit-parameter.t with the following commit: https://github.com/perl6/roast/commit/7192d95e4b and I'm closing this ticket.
[perl #114696] [BUG] $_ variable disappears after Rakudo sees block in for @array -> $variable {}
Christian Bartolomaeus via RT Wed, 15 Oct 2014 23:48:46 -0700