modifying readonly arrays

gistfile1.txt

use v6;

{
	my $x = 2;
	my @x = 2, 3, 4;
	say foo(@x);
	say bar($x);
}

sub foo(@z) {
	@z.pop; #is legal; why?
	return @z;
}

sub bar($z) {
	$z = $z + 2; #is illegal
	return $z;
}