Keep the common BadGuy stuff in a role, and compose that into different subclasses for the client and server.

package TheGame::Roles::BadGuy {
   use Moose::Role;
   has 'name' => (is => 'ro', isa => 'Str');
   requires 'health';
}

package TheGame::Client::BadGuy {
   use Moose;
   with 'TheGame::Roles::BadGuy';
   sub health {
      my $self   = shift;
      my $health = do { ... };
      return int($health/10)*10;  # round to nearest 10%
   }
}

package TheGame::Server::BadGuy {
   use Moose;
   with 'TheGame::Roles::BadGuy';
   has health    => (is => 'rw', isa => 'Num');
   has inventory => (is => 'rw', isa => 'ArrayRef');
}
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