You misunderstand my question. Here $self is a proper object, blessed. It seemed not too enlightening to list the new function of the object ( it is well known how to do it ). However here it is in full detail:

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;
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I want more than simply sorting the keys. Based on the keys I have to look up something within the object based on which I can sort the keys. For this I have to access the object's variables. Currently I create a global variable called $sort_self and I set this manually before calling the user defined cmp function byvalue, but this seems very unelegant.