Taking almut's (++) solution one step further, you could avoid the @tempd array entirely, using either a regular hash-ref assignment (for a new segment of the hash), or a hash-slice assignment:

 # Using hash-ref assignment:
   $tempa{a}{zeta}  =  {nm=>'abc', sz=>'def'};

 # Using Hash slice --
  @{$tempa{a}{zeta}}{qw[nm sz]} = qw[abc def];

#Resulting structure:
#= {
#    'a' => {
#             'zeta' => {
#                         'sz' => 'def',
#                         'nm' => 'abc'
#                       }
#           }
#  };
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Data::Dumper is your friend, in viewing these structures.