Actually I think you can do even simpler/easier to read:

use Exporter;
use base qw/Exporter/;
our %constants;

BEGIN {
  push @ISA, 'EXPORTER';
  %constants = (SUCCESS => 1);
  @EXPORT = map "&$_", keys %constants;
}

use constant \%constants;
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Since I guess your goal is to export all the constants in Demo2 without having to provide the list of names twice. And with the & appended to each name, there's no risk of overwriting $SUCCESS;