As well as getting an answer to my question I also set Abigail, Uri and Anno off on a discussion of why the original behaviour occured at all and wether or not it was a bug. (Not that I had been in any doubt at all).
The problem is your use of -1 as index on an empty array. An empty
array has no elements, so it certainly doesn't have a last
element. Due to Perl using 'call by implicit reference' when calling
functions with arguments, it must know which element you mean, just in
case foo assigns to $_[0].
sub foo { $_[0] = 1 }
my @q;
foo $q[2];
print scalar @q;
This prints 3 because you created $q[2]. But suppose
you had called
foo $q[-1] how large should @q be afterwards? Which element was
created?
Of course the subroutine call and the empty array are not actually relevant, you get the same if you do.
my @q = ( 'foo' ); my $r = \$q[-2];