I am trying my best to understand exactly how to add just a key without a value to a hash.

Although not actually very useful in the real world, this can be accomplished by simply assigning undef to a hash key. The key will then exist:

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";
}
[download]

What I am going to eventually shoot for, is making a hash of hashes

To accomplish this, you would make the value of your outer hash (of hashes) a reference to the inner hash, like so:

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";
[download]

Or you could use references all the way to begin with: (Notice the curly brackets)

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'}
);
[download]

---

---

Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!

Titles consisting of a single word are discouraged, and in most cases are disallowed outright.

Read Where should I post X? if you're not absolutely sure you're posting in the right place.

Please read these before you post! —

• How do I compose an effective node title?
• How do I post a question effectively?
• Markup in the Monastery

Posts may use any of the Perl Monks Approved HTML tags:

a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr

You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)

	For:		Use:
	&		&
	<		&lt;
	>		&gt;
	[		&#91;
	]		&#93;

Link using PerlMonks shortcuts! What shortcuts can I use for linking?

See Writeup Formatting Tips and other pages linked from there for more info.