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

my %HoH = (
0 => {
    b  => 2,
    a  => 1, 
    e  => 5,
    c  => 3,
    d  => 4,
}
);

foreach my $key (sort { $HoH{0}{$a} cmp $HoH{0}{$b} } keys $HoH{0}) {
    my $value = $HoH{0}{$key};
    print $value->{$key}, "\n";
}

##</code><code>##

1 -> a
2 -> b
3 -> c
4 -> d
5 -> e