A hash, or more technically an associative array, is a collection of key:value pairs. For example, let's say you want to create the following key:value pairs.

name: John Doe
id: 123456
department: engineering

You can create an empty hash and then append key:value pairs to the hash. This is the common approach. Or you can create a hash that contains key:value pairs.

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

my %hash;
my %hash = ();

key:value pairs can be appended to the hash, like this.

%hash = (
  name => "John Doe", 
  id => "123456", 
  department => "engineering"
);

Or, like this.

$hash{"name"}       = "John Doe";
$hash{"id"}         = "123456";
$hash{"department"} = "engineering";

An if statement can be used to confirm that the hash is not empty.

if (keys %hash) { 
  my $size = keys $hash;
  print "\%hash is not empty (contains $size keys)\n"; 
}

Dumper

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

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

Which will display the data structure of the hash.

$VAR1 = {
           'name' => 'John Doe',
           'id' => '123456',
           'department' => 'engineering'
        };

Or, you can print the value of a key. For example, the following will print the value in the "name" key (Jeremy).

print $hash{"name"};

Loop through keys / values

You will usually want to loop through the hash.

• Loop through hash keys
• Loop through hash values

Hash arrays

Each hash key will contain a single value. If you were to do this . . .

%hash = (
  name => "John Doe", 
  id => "123456", 
  department => "engineering"
);

%hash = (
  name => "Jane Doe", 
  id => "987654", 
  department => "sales"
);

Or this . . .

$hash{"name"} = "John Doe";
$hash{"name"} = "Jane Doe";

The hash would only contain the record for Jane Doe.

$VAR1 = {
           'name' => 'Jane Doe',
           'id' => '987654',
           'department' => 'sales'
        };

You can store the hash in an array. 

• Hash arrays
• Multidimensional hash arrays