[perl #113904] [BUG] Null PMC Access when blocks get the wrong $_ sometimes in Rakudo

[Carl Mäsak via RT]

<zjmarlow> r: my $line; for < one two three >.values -> $line { if True 
{ .say } }
<p6eval> rakudo bf472b: OUTPUT«Null PMC access [...]
<masak> huh.
<masak> we may have this in RT already, actually.
<masak> r: for 1 -> $ { if 1 { .say } }
<p6eval> rakudo bf472b: OUTPUT«Null PMC access [...]
<masak> yes, here: https://rt.perl.org/rt3/Ticket/Display.html?id=113904
<masak> though this particular case has an 'if', not a 'when'.
<masak> r: $_ = 42; .say
<p6eval> rakudo bf472b: OUTPUT«42␤»
<masak> r: $_ = 42; for 1 { .say }
<p6eval> rakudo bf472b: OUTPUT«1␤»
<masak> r: $_ = 42; for 1 { if 1 { .say } }
<p6eval> rakudo bf472b: OUTPUT«1␤»
<masak> r: $_ = 42; for 1 -> $p { .say }
<p6eval> rakudo bf472b: OUTPUT«42␤»
<masak> r: $_ = 42; for 1 -> $p { if 1 { .say } }
<p6eval> rakudo bf472b: OUTPUT«Null PMC access [...]
<masak> this one illustrates it fairly well. I'll paste it into the 
ticket.