[Original code deleted; see update below]

##</code><code>##

sub get_cog {
  my ($self) = @_;
  my $class = ref $self;

  if ( $class eq "Thingy" ) {
    return Thingy::Cog->new;
  }
  else {
    my $cog_class = "$class\::Cog";
    (my $cog_class_file = $cog_class . "pm") =~ s{::}{/};

    if ( grep { -e $_ } map { $_/$cog_class_file } @INC ) {
      require $cog_class_file;
      return $cog_class->new;
    }
    else {
      return Thingy::Cog->new;
    }
  }
}