package BossClass;
  ...
sub new {
  ...
}
sub create {
  my $obj = $a ? Aclass->new(...) : 
            $b ? Bclass->new(...) : 
            Cclass->new(...);
  my $res = $obj->do_it(...);
  ...
}

##</code><code>##

use Test::More;
use Test::Resub qw/ resub /;

my $mocker = resub 'Aclass::do_it', sub { }, capture=>1;
my $tobj = BossClass->new(...);
$tobj->create();
my $args = $mocker->args;
# Tests check the args BossClass provided to the chosen constructor
# and the do_it method are as expected