package ParentClass;

use base qw(Exporter); # requires and adds Exporter to @ISA
BEGIN { our @EXPORT = qw(FOO); } # what must be known elsewhere

use constant FOO => 'foo';

package ChildClass;

use base 'ParentClass';
BEGIN { P->import(); } # import FOO

sub doStuff {
  my ($self, @otherArguments) = @_;

  print FOO; # should work just fine
  print $self->FOO; # just the same as above

}