Since BrowserUk has already provided the answer, this reply is very much BTW, but FWIW...

In the OPed code fragment, the statement
    $practice->{$patient->{suregery_id}} = \$patient;
repeatedly assigns (i.e., overwrites) a reference to  $patient to the hash key  $patient->{suregery_id} of the hash referenced by  $practice.

The OPed code iterates over an array of hash references, so  $patient is already a hash reference. The statement above repeatedly assigns the LHS hash element a reference to a reference to a hash, which is probably even less what weezer_316 wanted! (Again, not that it matters much at this point.)