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my %hash = ();
$hash{'foo'} = undef;
...
} else {
  print "The key 'foo' is false.\n";
}

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";

my $entry = { firstname => 'Ola', lastname => 'Nordmann' }; 
my $staff = {};
...
  $staff->{$id}->{'lastname'}, 
  $staff->{$id}->{'firstname'}
);