package Base;
# define default values
sub earthbound { 0 }
sub metallic { 0 }
sub blood { "cold" }
sub weakness { undef }

# update 2:
sub validate {
  my ($class) = @_;
  return ($class->blood eq 'warm' or ! $class->metallic);
}


# define 'mixins' - don't subclass these,
# they'll export their properties when you use them

package Flies;
use base Exporter;
our @EXPORT = qw(earthbound);
sub earthbound { 1 }

package Robot;
use base Exporter;
our @EXPORT = qw(metallic);
sub metallic { 1 }

# etc...

package Superman;
use base Base;
use Flies;
use Robot;  # superman's very metallic.