The fact is that your x= tricks works as just as much as the undef() because of undocumented behaviour. This is shown by 

my $foo;
sub context :lvalue { $foo = wantarray ? 'list' : 'scalar' } 

context() x= 1;
print $foo;

__END__
scalar

[download]
or using simpler means: 
use Data::Dumper;

my %foo = qw/ a 1 b 2 /;
@foo{qw/ a b /} x= 0;
print Dumper \%foo;

__END__
$VAR1 = {
          'a' => '1',
          'b' => ''
        };

[download]
A trick that does work though is the reference trick. All these other solutions work because of autovivification. So let's use \ whose behaviour explicitly is documented. 
\@foo{@list};

[download]

ihb

See perltoc if you don't know which perldoc to read!

In reply to Re^5: Adding Unique Elements to Array by ihb
in thread Adding Unique Elements to Array by thekestrel

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