Question

Yet another data structure doubt. I will get straight to it.
This is what I have

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

my $head= undef;
my $tail=\$head;

open FILE, "<datastored.txt" or die $!;

while (<FILE>){
    my $node = {
                "data" => $_ , 
                "next" => undef
            };
    $$tail=$node; 
    $tail = \${$node->{"next"}}; 
};

# #1# Print full list #
print Dumper $head;

# #2# Delete the first node and display data of next node #
$head = $head->{next};
my $value = $head->{data};

and this is the output I get

$VAR1 = {
          'next' => \{
                        'next' => \{
                                      'next' => \{
                                                    'next' => \undef,
                                                    'data' => 'line 4'
                                                  },
                                      'data' => 'line 3
'
                                    },
                        'data' => 'line 2
'
                      },
          'data' => 'line 1
'
        };
Not a HASH reference at linkedlist.pl line 32, <FILE> line 4. **<<<< My Problem and hence my question?** 

Note – the content of the file datastored.txt is simply

line 1
line 2
line 3
line 4

If I look at the print result of $head initially, it clearly is a hash within a hash and so on, so why is that after deleting the first node is the integrity of the data structure hampered? (See Error on last line of output)

Answer 1

Perl references are tricky. The dumper output notation \{ is showing that the {next} fields contain references to references to hashes.

The offending line is the update to tail. Try

$tail = \$node->{"next"};