Package MyObject;

sub new
{

my $self={
hash_ref=>{key1=>'value1',key2=>'value2'}
};

bless $self;
return $self;

}

#global variable to access $self

my $sort_self;

sub byvalue
{
$sort_self->{hash_ref}->{$a}<=>$sort_self->{hash_ref}->{$b};
}

sub sort_hash_ref_keys_by_value
{
my $self=shift;

#set global variable by hand
$sort_self=$self;

#now the cmp function knows the object
my @keys_sorted_by_value=sort byvalue keys(%{$self->{hash_ref}});

}

Packahe Main;

my $myobject=new MyObject;

$myobject->sort_hash_ref_keys_by_value;