sub double {
    sub { \@_ }->($_[0], $_[0])
}
my $r = double my $v;
$v = 1;
@$r; # => ( 1, 1 );
$v = 2;
@$r; # => ( 2, 2 );
$r->[0] = 3;
@$r; # => ( 3, 3 );