package ParentClass;
use constant FOO => 'foo';
...
package ChildClass;
use base 'ParentClass';
sub doStuff {
  my ($self, @otherArguments) = @_;
  ...
  #print FOO; # dang! this generates the 'Bareword "EMPTY_FLAG" not allowed...' error
  print $self->FOO;
  ...
}