[perl #80614] [Bug] chained array item assignment leaves one item undefined

[Christian Bartolomaeus via RT]

I tried to golf this (chained assignment and test failure) a bit and found the 
following:

$ perl6-m -e 'my $x; $x = 1; my @a; my $y = @a[0] = 2; say @a[0]'
2
$ perl6-m -e 'my $x; ($x) = 1; my @a; my $y = @a[0] = 2; say @a[0]'
(Any)

The following looks weird as well:

$ perl6-m -e 'my $y; my @a = (); $y = @a[0] = 2; say @a[0]'
2
$ perl6-m -e 'my $y; my @a; @a = (); $y = @a[0] = 2; say @a[0]'
(Any)