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

Generally speaking, there are two approaches to create the hash key:value pair. You either create an empty hash and then append values (this is the much more common approach) or you define the hash and the key:value pairs.

Following are both valid examples of how to create an empty hash (does not contain any key value pairs).

my %hash;
my %hash = ();

An if statement can be used to confirm that the hash is defined.

if (%hash) { print "\%hash is defined\n"; }

An if statement can be used to confirm that hash is empty (contains no key).

if (not keys %hash) { print "\%hash is empty (contains no keys)\n"; }

Now, key:value pairs can be created in the hash. Or, this could be one line. I just put the key value pairs on new lines for readability.

my %hash = (
  name => Jeremy, 
  id => 123456, 
  occupation => Engineer
);

Or, one at a time.

$hash{name}       = "Jeremy";
$hash{id}         = "123456";
$hash{occupation} = "Engineer";

An if statement can be used to confirm that the hash is not empty (contains at least one key:value pair).

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

Printing

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

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

Which will print all of the keys and values in the hash, like this.

$VAR1 = {
           'name' => 'Jeremy',
           'id' => '123456',
           'occupation' => 'Engineer'
        };

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

print $hash{name};

Loop through keys / values

You will usually want to loop through the hash. The following foreach loop will print the keys.

foreach my $key (keys %hash) {
    print $key;
}

Which will print each key.

name
id
occupation

The following foreach loop will print the value associated with each key.

foreach my $key (keys %hash) {
    print $hash{$key};
}

Which will print each value.

Jeremy Canfield
123456
Engineer

Or like this.

foreach my $value (values %hash) {
    print $value;
}

Will print the values.

Jeremy Canfield
123456
Engineer

Or like this, to loop through both the keys and the values.

foreach my $key (keys %food) {
  foreach my $value ($hash{$key}) {
    print "$key -> $value\n";
  }
}

Will print the following.

name -> Jeremy Canfield
id -> 123456
occupation -> Engineer

Arrays

Just like variables, each hash key will contain a single value. If you were to do something like this, $hash{name} will only contain a value of Winston.

$hash{name} = "Jeremy";
$hash{name} = "Winston";

You can store the hash in an array. In this example, the @{$hash{name}} array will contain both Jeremy and Winston.

push ( @{$hash{name}}, "Jeremy" );
push ( @{$hash{name}}, "Winston" );

You can then use two foreach loops to print the key:value pairs in the array.

foreach my $key (keys %hash) {
  foreach my $value (@{$hash{$key}}) {
    print "$key = $value\n";
  }
}

Which will print the following.

name = Jeremy
name = Winston

Or use Dumper to display the structure.

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

Which will produce the following.

$VAR1 = {
          'name' => [
                      'Jeremy',
                      'Winston'
                    ]
         };