package Super::Class;

my $idx = 0;

sub NEXT_IDX { return $idx++ };

use constant FOO_IDX => NEXT_IDX();
use constant BAR_IDX => FOO_IDX + 1;
use constant BAZ_IDX => BAZ_IDX + 2;

package Sub::Class;
# need to wrap in BEGIN blocks so SUPER:: works in the use at
# compile time.
BEGIN {our @ISA = qw(Super::Class)};

use constant OTHER_IDX => __PACKAGE__>SUPER::NEXT_IDX();
# oops! NEXT_IDX() is still set to '0' up there, and we just set
# OTHER_IDX to 1, clobbering whatever's in the BAR_IDX field!