if ($myHash{unknown}{if}{this}{key}{combination}{exists}{yet}{or}{not}
){...}
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Unless this is some kind of data structure that you have no control over, You're Doing It Wrong.

If something is really that deeply nested - and might not exist - I'd say that there is a real architectural problem.

Something else to take a look at is Data::DPath which might render your code as simple as this:

use Data::DPath 'dpath';

if( dpath('/unknown/if/this/key/combination/exists/yet/or/not') ) {
  # Yay
}
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