#!/usr/bin/perl

$list = undef;
foreach (reverse 1..5) {
    $list = [ $list, $_ * $_ ];
}

use constant NEXT => 0;
use constant VAL  => 1;

$four    = $list->[NEXT];
$nine    = $four->[NEXT];
$sixteen = $nine->[NEXT];

$nine->[NEXT]    = $sixteen->[NEXT];
$sixteen->[NEXT] = $nine;
$four->[NEXT]    = $sixteen;