#=================
package BaseClass;
#=================
our $ID = 1;

sub new {
    my $class = shift;
    return bless {}, $class;
}

sub id {
    my $self  = shift;
    my $class = ref $self;

    return eval "\$${class}::ID";
}

#================
package SubClass;
#================
use base qw( BaseClass );
our $ID = 2;

#============
package main;
#============
my $obj = new SubClass;
my $id  = $obj->id();

print "$id\n";