use Data::Dumper;
sub nothing { return () }
my %hash = (
   state => 'db_user',
   usr_id => 1,
   edit_usr_id => nothing(),
   mode => 'add',
);
warn Dumper\%hash;

%hash = (
   state => 'db_user',
   usr_id => 1,
   edit_usr_id => scalar nothing(),
   mode => 'add',
);
warn Dumper\%hash;
__DATA__
$VAR1 = {
          'add' => undef,
          'usr_id' => 1,
          'edit_usr_id' => 'mode',
          'state' => 'db_user'
        };
$VAR1 = {
          'usr_id' => 1,
          'mode' => 'add',
          'edit_usr_id' => undef,
          'state' => 'db_user'
        };