You can declare the keys as constants as you suggest, or use the lock_keys function from Hash::Util, which comes standard in recent perls. The latter will raise an exception at runtime if an non-existant key is set.

BTW, if you do use constants, an array is more efficient unless only a sparse set of keys are ever used at once:

use constant { K_AGE => 0, K_NAME => 1 };

my @record;
   $record[K_AGE]  = 1001;
   $record[K_NAME] = 'Doraemon';
[download]

The difficulty in making hashes "strict" is just one of the reasons why many people dislike blessed-hashref object implementations.

Update: See also: non-autovivifing hash, how to avoid mis-spelling hash keys?.

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