sub insert_after {
	my ($previous, $val) = @_;
	my $curr_next = $previous->{next};
	$previous->{next} = {
                "data" => $val , 
                "next" => $curr_next,
                "prev" => $previous,
            };
    my $new_next = $previous->{next};
    $curr_next->{prev} = $new_next if defined $curr_next;
    return $new_next;
}	

sub insert_before {
	# simply call insert_after on the previous node
	my ($previous, $val) = @_;
	return insert_after($previous->{prev}, $val);
}

##</code><code>##

sub display {
	my $curr = shift;
	while (1) {
		last unless defined $curr->{next};
		print "Current: ", $curr->{data}, 
		      "\tPrevious: ", $curr->{prev}{data} // "", 
		      "\tNext: ", $curr->{next}{data} // "", "\n";
		$curr = $curr->{next};
	}
    print "\n";
}

##</code><code>##

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

while (<DATA>){
    # ... same as in previous post
}

display($head);
my $new_node = insert_after ($head, 0.5);
$new_node = insert_before ($new_node->{next}, 0.7);
display($head);

##</code><code>##

$ perl  doubly_linked.pl
Current: 0      Previous:       Next: 1
Current: 1      Previous: 0     Next: 2
Current: 2      Previous: 1     Next: 3
Current: 3      Previous: 2     Next: 4
Current: 4      Previous: 3     Next: 5

Current: 0      Previous:       Next: 0.5
Current: 0.5    Previous: 0     Next: 0.7
Current: 0.7    Previous: 0.5   Next: 1
Current: 1      Previous: 0.7   Next: 2
Current: 2      Previous: 1     Next: 3
Current: 3      Previous: 2     Next: 4
Current: 4      Previous: 3     Next: 5


##</code><code>##

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

while (<DATA>){
    chomp;
    $tail = insert_after($tail, $_);
}

##</code><code>##

#!/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);


##</code><code>##


$ perl  doubly_linked.pl
Current: 0      Previous:       Next: 1
Current: 1      Previous: 0     Next: 2
Current: 2      Previous: 1     Next: 3
Current: 3      Previous: 2     Next: 4
Current: 4      Previous: 3     Next: 5

Current: 0      Previous:       Next: 0.5
Current: 0.5    Previous: 0     Next: 0.7
Current: 0.7    Previous: 0.5   Next: 1
Current: 1      Previous: 0.7   Next: 2
Current: 2      Previous: 1     Next: 3
Current: 3      Previous: 2     Next: 4
Current: 4      Previous: 3     Next: 4.5
Current: 4.5    Previous: 4     Next: 5