$c = ['c', undef];
$b = ['b', undef];
$a = ['a', undef];

$a->[1] = $b;
$b->[1] = $c;

##</code><code>##

my $current = $a;

while (defined $current) {
    my $value = $current->[0];
    ...
    $current = $current->{1];
}