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

use Data::Dumper;

my $self = {
            hash_ref => { item1 => 12,
                          item2 =>  9,
                        },
           };

print Dumper(sort_hash_ref_keys_by_value($self));

sub sort_hash_ref_keys_by_value {
    my $self = shift;
    my $by_value = sub {
        $self->{hash_ref}->{$a} <=> $self->{hash_ref}->{$b}
    };
    return sort $by_value keys %{ $self->{hash_ref} };
}