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 );

   ...
}