use strict;
use warnings;
use Hash::Util qw( lock_keys unlock_keys );

my %hash = ( 'name' => "Doraemon", 
             'age'  => "1000"
           );

lock_keys( %hash );

# oh! my birthday, i'm getting older
$hash{'ege'} = "1001";  # oops, just TRIED to add new key.
[download]

...and the output...

Attempt to access disallowed key 'ege' in a restricted hash at 
C:\Perl\scripts\hashtest.pl line 14.
[download]

Enjoy!