In Perl, there are 2 different kinds of hashes.

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

In this example, the foo key in the hash is an array, as indicated by the [ ] characters. The foo key is empty (contains no values).

my %hash = ( 'foo' => [] );

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

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

Which should produce the following.

$VAR1 = {
          'foo' => []
        };

Let's say you do this.

push @{$hash{foo}}, { 'name' => 'John Doe', 'department' => 'engineering' };
push @{$hash{foo}}, { 'name' => 'Jane Doe', 'department' => 'sales' };

Now, the foo key contains key value pairs.

$VAR1 = {
          'foo' => [ 
                     {
                       'name' => 'John Doe', 
                       'department' => 'engineering'
                     },
                     {
                       'name' => 'Jane Doe',
                       'department' => 'sales'
                     }
                   ]
        };

You can loop through the foo key to print the value associated with the key.

foreach my $foo (@{$hash{foo}}) {
  print "$foo->{name} \n";
}

Which in this example will print the value of the name key.

John Doe
Jane Doe

Let's say the hash already contains a key value pair, like this, and you want to append another key value pair at the same place in the hash.

$VAR1 = {
          'foo' => [ 
                     {
                       'foo' => 'John Doe'
                     }
                   ]
        };

The following will appended a new key value pair to the block that contains John Doe.

foreach my $foo (@{$hash{foo}}) {
  if ($foo->{name} eq "John Doe") {
    $foo->{department} = "engineering";  
  }
}

Now the hash looks like this.

$VAR1 = {
          'foo' => [ 
                     {
                       'name' => 'John Doe', 
                       'department' => 'engineering'
                     }
                   ]
        };

Let's say you want to create an array like this, where the bar key is a child of the foo key, meaning the hash is multidimensional.

$VAR1 = {
          'foo' => [ 
                     {
                       'bar' => [
                                  {
                                    'name' => 'John Doe',
                                    'role' => 'Engineer'
                                  }
                                ]
                     }
                   ]
        };

AVOID TROUBLE

The hash must be defined before key value pairs can be appended to the bar key.

Define the hash.

my %hash = ( 'foo' => [ { 'bar' => [] } ] );

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

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

Which should return something like this.

$VAR1 = {
          'foo' => [ 
                     {
                       'bar' => []
                     }
                   ]
        };

You can loop through the top level key (foo) and print the Dumper output.

foreach my $foo (@{$hash{foo}}) {
  print Dumper $foo;
}

Which should produce the following.

$VAR1 = {
          'bar' => []
        };

Here is how to append key value pairs to the bar array.

foreach my $foo (@{$hash{foo}}) {
  push @{$foo->{bar}}, { 'name' => 'John Doe', 'role' => 'Engineeer' };
}

Now Dumper should return the following.

$VAR1 = {
          'foo' => [ 
                     {
                       'bar' => [
                                  {
                                    'name' => 'John Doe',
                                    'role' => 'Engineer'
                                  }
                                ]
                     }
                   ]
        };

You can loop over the foo key.

foreach my $foo (@{$hash{foo}}) {
  print Dumper $foo;
}

Which should return the following.

$VAR1 = {
          'bar' => [
                     {
                       'name' => 'John Doe',
                       'role' => 'Engineer'
                     }
                   ]
        };

You can loop over the parent key (foo) and print the child keys (bar).

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

Which should return the following.

$VAR1 = 'bar';

You can loop over the parent key (foo) and print the values from the child key (bar).

foreach my $foo (@{$hash{foo}}) {
  foreach my $value (values $foo) {
    print Dumper $value;
  }
}

Which should return the following.

$VAR1 = [
          {
            'name' => 'John Doe',
            'role' => 'Engineer'
          }
        ];

Or like this.

foreach my $foo (@{$hash{foo}}) {
  foreach my $value (@{$foo->{bar}}) {
    print "$value->{name}\n";
    print "$value->{role}\n";
  }
}

Which should return the following.

John Doe
Engineer