Holding refs to subroutines seems like a perfectly good way to do private methods:

package Foo;
                                                                      
. . . 

my $fiddle = sub {
    my $self = shift;
    return map { scalar reverse $_ } @_;
};

sub wonk 
{
    my $self = shift;
    $self->$fiddle(@_);
}


package Bar;

sub wonk 
{
    my $self = shift;
    $self->$fiddle(@_);  # runtime error
}
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The real problem is that Perl doesn't have a good way of denoting protected methods or attributes. I don't consider litering your code with $obj->isa( . . . ) or die; to be a good way.