use strict;
use warnings;

package Foo;
use constant X => 100;
sub new {  bless([], $_[0]); }

package Goo;
our @ISA=qw(Foo);
use constant X => 200;
sub new {  bless([], $_[0]); }

package MeToo;
our @ISA=qw(Foo);
sub X { print "Hello World\n"; }


my $oFoo = Foo->new();
my $oGoo = Goo->new();
my $oMeToo = MeToo->new();

print "$oFoo -> X = ", $oFoo->X, "\n";
print "$oGoo -> X = ", $oGoo->X, "\n";
$oMeToo->X();

#outputs
Foo=ARRAY(0x8218ed0) -> X = 100
Goo=ARRAY(0x8218f10) -> X = 200
Hello World