I'm not sure how helpful this will be towards your end goal but you might want to check out miyagawa's Attribute::Protected which implements C++/Java style encapsulation of methods (pity it doesn't do package level encapsulation though ;-). It might go part of the way to protecting My::B to the desired level.

# NOTE: slightly modified code from the docs

package SomeClass;
use Attribute::Protected;

sub new { return bless {}, shift }

sub foo  : Public    { }
sub _bar : Private   { }
sub _baz : Protected { }

sub another {
      my $self = shift;
      $self->foo;               # OK
      $self->_bar;              # OK
      $self->_baz;              # OK
}

1;

package DerivedClass;
  @DerivedClass::ISA = qw(SomeClass);

sub yetanother {
      my $self = shift;
      $self->foo;               # OK
      $self->_bar;              # NG: private method
      $self->_baz;              # OK
}

1;

package main;

my $some = SomeClass->new;
   $some->foo;          # OK
   $some->_bar;         # NG: private method
   $some->_baz;         # NG: protected method
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HTH

broquaint