In Perl, there are 3 different kinds of hashes.

• A regular hash, which is defined by the % and ( ) characters - %hash = ( );
• A reference hash, which is defined with the $ and { } characters = $hash = { };
• A hash array, which is defined with the % and [ ] characters - %hash = [ ];

Here are different ways to create an empty hash named "hash".

my %hash;
my %hash = ();

Dumper can be used to display the structure of the hash.

use Data::Dumper;
print Dumper \%hash;

Which should produce the following.

$VAR1 = {};

Keys that contain no values (empty keys) can be created like this.

%hash = (
  foo => "", 
  bar => ""
);

Or, like this.

$hash{foo} = undef;
$hash{bar} = undef;

Or, like this.

$hash{foo} = "";
$hash{bar} = "";

Dumper can be used to display the structure of the hash.

print Dumper \%hash;

Which will display the data structure of the hash.

$VAR1 = {
           'foo' => '',
           'bar' => ''
        };