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package C::C1; # That's the perlish name for the module
use File::FindLib 'lib';
...
}
# etc
# Repeat for C and D, ad libidum.

sub new_d {
    my $self = shift;
...
    }
    return $self->{_d};
}

use C::C1;
my $c1 = C::C1->new;
$c1->a_method_1();   # will execute A::a_method_1
$c1->b_method_1();   # will execute B::b_method_1
$c1->new_d(@params)  # will execute $self->{_d} = D->new(@params)