#!/usr/bin/perl

use strict;
use warnings;
use Data::Dumper;

my $head = {data => 0, prev => undef, next => undef};
my $tail = $head;

while (<DATA>){
    chomp;
    $tail = insert_after($tail, $_); # new tail returned each time through
}

display($head);
my $new_node = insert_after ($head, 0.5);
$new_node = insert_before ($new_node->{next}, 0.7);
# adding a new node before the tail
$new_node = insert_before ($tail, 4.5);
display($head);