DB<105> push @{ $hash{A1}{B2}{C1} }, "arrayitem4";
 => 1

  DB<106> \%hash
 => { A1 => { B2 => { C1 => ["arrayitem4"] } } }

  DB<107> push @{ $hash{A1}{B2}{C1} }, "arrayitem1";
 => 2

  DB<108> \%hash
 => { A1 => { B2 => { C1 => ["arrayitem4", "arrayitem1"] } } }

  DB<109> push @{ $hash{A1}{B2}{C2} }, "arrayitem2";
 => 1

  DB<110> \%hash
 => {
  A1 => {
          B2 => { C1 => ["arrayitem4", "arrayitem1"], C2 => ["arrayitem2"] },
        },
}