my %hash = ( 'A1' => { 
                               'B1' => 'Value',
                               'B2' => { 
                                        'C1' => [arrayitem1, 
                                                 arrayitem2,
                                                 arrayitem3,],

                                        'C2' => [arrayitem1, 
                                                 arrayitem2,
                                                 arrayitem3,],
                                       }
                              },
                     )
##</code><code>##

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