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.

Start with a hash that contains values

In this example, a hash named %hash is created, and %hash contains the key:value pairs.

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

Start with an empty hash

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

my %hash;
my %hash = ();

Then, somewhere in the script key:value pairs will be created. In this example, a key called "name" is created with a value of "Jeremy" and another key called "occupation" with a value of "engineer".

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

Printing

You can print the contents of the entire hash.

print %hash;

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

Name Jeremy Canfield 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

Arrays

Each hash key will contain a single value, which is to say that a hash key cannot contain multiple values. For example, if you were to do something like this,  $hash{name} will only contain a value of Winston.

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

A solution to this is to create an array that contains multiple values. 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 a foreach loop to print all of the values in the array.

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