#!/usr/bin/perl
# indexhash.pl
use warnings;
use strict;

#create a hash with integrated index
my %hash = {
	Mouse => {Index => 0, Value => 'Jerry'},
	Cat   => {Index => 1, Value => 'Tom'},
	Dog   => {Index => 2, Value => 'Spike'}
	};

# sort a hash by integrated index
foreach (sort {$hash{$a} {'Index'} cmp $hash{$b} {'Index'}} keys %hash)  
	{
	print "$hash{$_} {'Value'} <= $_ \n";
	}

##</code><code>##

Reference found where even-sized list expected at indexhash.pl line 7.
Use of uninitialized value in concatenatization (.) or string at indexhash.pl line 16.