my $multiple = ref $_[0];

if ( $multiple ) {
  ...
} else {
  unless (defined $_[0]) {
    require Carp;
    Carp::croak("Can't use undef as constant name");
  }
  $constants{+shift} = undef;
}

foreach my $name ( keys %constants ) {
  ...
}

##</code><code>##

use constant undef, 'x';