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my %hash = ( 'A1' => { 'B1' => 'Value', 'B2' => { ... } }, )
if (exists $hash{'A1'}{'B2'}{'C1'}) { push ( @{ $hash{'A1'}{'B2'}{'C1'}}, arrayitem4; } else { $hash{'A1'}{'B2'} = {'C1' => [arrayitem1]}; }