use strict;
use warnings;
use Data::Dump 'pp';

my %hash;

$hash{tree} = [4];
push @{$hash{tree}}, {apple => [6]};

pp \%hash;

my $apple_tree = $hash{tree}->[1]->{apple};
push @$apple_tree, {red => 9, fuju => 4};

pp \%hash;

__END__
{ tree => [4, { apple => [6] }] }
{ tree => [4, { apple => [6, { fuju => 4, red => 9 }] }] }