package Thingy;
sub make_cog {
   my $self = shift;
   return Cog->new(@_);
}

{
   ...
   my $cog = $self->make_cog(...);
   ...
}

package MyThingy;
sub make_cog {
   my $self = shift;
   return MyCog->new(@_);
}

##</code><code>##

package Thingy;
sub make_cog {
   my $self = shift;
   my $cog_class = ref($self) . '::Cog';
   if ($cog_class->can('new')) {
      return $cog_class->new(@_);
   } else {
      return Thingy::Cog->new(@_);
   }
}

##</code><code>##

sub make_class {
   my ($class, $base_class) = @_;

   my $isa_glob = do { no strict 'refs'; ${"${class}::"}{ISA} };
   if (!$isa_glob || !*$isa_glob{ARRAY}) {
      @$isa = $base_class;
   }
}

{
   my $group = 'MyThingy';

   make_class("${group}::$_', 'Thingy::$_')
      foreach qw( Cog Ramp Handle );

   ...
}