use strict;
use warnings;

$|=1;


sub empty_list {
  @{ [] }
}

sub use_hash {
  my (%h) = @_;
  print(join(',',keys %h),"\n");
}

use_hash(foo1 => undef, bar1 => 5);

print "not defined\n" unless defined ((empty_list)[0]);

use_hash(foo2 => (empty_list)[0], bar2 => 5);