package MyClass;

{
    # we make an attribute hash private to the foo method
    my %foo;
    sub foo {
        my $self = shift;
        $foo{$self} = shift if @_;
        return $foo{$self};
    };
};

sub bar {
    my $self = shift;
    # so we can only access %foo using method foo here
    print $self->foo;
};