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 = { };

Let's say you have the following hash. In this example, the foo key in the hash is empty.

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

A value can be appended to the employee key, like this.

$hash{foo} = "bar";

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

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

Now, the employee key contains a value of John Doe.

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

The lock_keys and lock_value subroutines can be used so that the value in the key cannot be modified.

use Hash::Util qw (lock_keys unlock_keys lock_value unlock_value);

In this example, every key in the hash is locked, so that no keys can be added, modified, or removed. 

lock_keys (%hash);

The value with the employee key is locked, so that the value cannot be modified or removed.

lock_value(%hash, "foo");

Attempting to modify the value of a locked key value pair . . .

$hash{foo} = "Hello World";

. . . will produce the following.

Modification of a read-only value attempted

The value can be unlocked.

unlock_value(%hash, "foo");

Every key in the hash can be unlocked, so that keys can be added, modified, or removed. 

unlock_keys (%hash);