Thank you all the monks who have replied with so much explation.
Today I was reading Programmaing Perl, 3rd Edition by Larry Wall, Tom Christiansen and Jon Orwant and found out a way to modify the code which I posted in my previous mail to implement private method. The modified code is

{
    package Animal;
    use Carp;
    use overload();

    sub new
    {
        my $class = shift;
        my $name = shift;
        bless \$name, $class;
    }

    sub name 
    {
        my $self = shift;
        ref $self ? $$self : "an unnamed $self";
    }

    sub speak 
    {
        my $class = shift;
        print "a $$class goes ", $class->sound, "!\n";
    }

    sub eat 
    {
        my $self = shift;
        my $food = shift;
        print $self->name, " eats $food.\n";
    }

    my $private_method = sub {
        my $self = shift;
        print "From private method\n";
    };

    sub Private_Method
    {
        my $self = shift;
        print "kk::$self->{private_var}\n";
        if (0 == index overload::StrVal($self), __PACKAGE__,0)
        {
            $self->$private_method();
        }
    }
}

{
    package Horse;
    use vars '@ISA';

    @ISA = qw (Animal);
    sub sound {"neigh"};

}

{    
    package Sheep;
    use vars '@ISA';
    @ISA = qw (Animal);
    sub sound {"blahhhhhhhh"};
}

my $horse1 = Horse->new("H1");
$horse1->speak;
print "Horse sounds ", Horse->sound, "\n";
$horse1->eat("hay");
Sheep->eat("grass");
Sheep->speak();
#$horse1->Private_Method; #I can not access this
Animal->Private_Method;
[download]

Is this method OK? Is there any loophole? Any help will be really appreciated.
Thanks in advance.
-Pijush