my %hash = ();
$hash{'foo'} = undef;

if (exists $hash{'foo'}) {
  print "The key 'foo' exists.\n";
} else {
  print "The key 'foo' does not exist.\n";
}

if (defined $hash{'foo'}) {
  print "The key 'foo' is defined.\n";
} else {
  print "The key 'foo' is undefined.\n";
}

if ($hash{'foo'}) {
  print "The key 'foo' is true.\n";
} else {
  print "The key 'foo' is false.\n";
}

##</code><code>##

my %inner_hash = ( dir => '/tmp/foo/', filename => 'bar.txt' );
my %hash_of_hashes = ();

$hash_of_hashes{'baz'} = \%inner_hash; # Backslash = reference to

print "The filename associated with 'baz' is "  .$hash_of_hashes{'baz'}->{'filename'} . "\n";

##</code><code>##

my $entry = { firstname => 'Ola', lastname => 'Nordmann' }; 
my $staff = {};

my $id = 123;
$staff->{$id} = $entry;

printf(
  "%d: %s, %s\n", 
  $id, 
  $staff->{$id}->{'lastname'}, 
  $staff->{$id}->{'firstname'}
);