#!/usr/bin/perl
use warnings;
use strict;

use Data::Dumper;

my @AoH = ({key1 => 'value1',
            key2 => 'value2',
            key3 => [ { var1 => 'val1',
                        var2 => 'val2',
                      },
                      #NEED TO ADD ANOTHER HASH HERE
                    ],
           },
          );

push @{ $AoH[0]{key3} }, { key => 'value' };
#    ^       ^   ^
#    |       |   |
#  deref    0th  |
#  as an    hash |
#  array         key in
#                the hash

$AoH[0]{key3}[2]{another} = 'way'; # if you know the index.

print Dumper \@AoH;