* . I don't want Package B to implement method a3( ).

You could override it. Simple as can be, and very straight forward.

package B;
use base 'A';
use Carp;
use strict;

sub a3 {
    croak
        sprintf q[Can't locate object method "a3" via package "%s"], 
        ref shift;
}
[download]

Please, no recommendations to refactor my classes.

But I really think you should.

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