[perl #120397] [BUG] Segfault (Moar, JVM) and Null PMC access (Parrot) when assigning to a variable "declared" in a subsignature in a variable declaration in Rakudo

[Christian Bartolomaeus via RT]

Status update: Right now this is no longer a Null PMC access error on Parrot 
but we get a segfault on Moar as well:

$ perl6-m -e 'my $a ($b, $c); $b = 42;'
Segmentation fault

$ perl6-p -e 'my $a ($b, $c); $b = 42;'
Cannot assign to an immutable value
  in block <unit> at -e:1

$ perl6-j -e 'my $a ($b, $c); $b = 42;'
java.lang.NullPointerException
  in block <unit> at -e:1