my %HOH;
$HOH{parent}{child1} = 'value1';
push @{ $HOH{parent}{child2} }, 'value2';
push @{ $HOH{parent}{child2} }, 'value3';


###
output:
{
  parent => { 
    child1 => "value1",
    child2 => ["value2", "value3"] },
}