NetWallah, I think there is no difference between your's and mine.

#!/usr/bin/perl
use strict;
use Data::Dumper;

my @AoH = ({
              title   => 'aaa',
              heading => [
                          {item  => 'bbb'},
                          {item  => 'ccc'},
                          {item  => 'ddd'},
            ]});

#$AoH[0]->{heading}->[3] = {item => 'eee'}; #your's
$AoH[0]->{heading}->[3]->{item} = 'eee';#mine

#print $AoH[0]->{title};
#print $AoH[0]->{heading}->[0]->{item};
#print $AoH[0]->{heading}->[1]->{item};
#print $AoH[0]->{heading}->[2]->{item};
#print $AoH[0]->{heading}->[3]->{item};

print Dumper @AoH;
[download]

your output:

$VAR1 = {
          'heading' => [
                         {
                           'item' => 'bbb'
                         },
                         {
                           'item' => 'ccc'
                         },
                         {
                           'item' => 'ddd'
                         },
                         {
                           'item' => 'eee'
                         }
                       ],
          'title' => 'aaa'
        };
[download]

my output:

$VAR1 = {
          'heading' => [
                         {
                           'item' => 'bbb'
                         },
                         {
                           'item' => 'ccc'
                         },
                         {
                           'item' => 'ddd'
                         },
                         {
                           'item' => 'eee'
                         }
                       ],
          'title' => 'aaa'
        };
[download]