For completeness:

if it's only the key db which matters

  DB<131> $conf3 = { db => { %{$conf1->{db}}, %{$conf2->{db}} } } 
=> 
{
  db => {
          host     => "localhost",
          name     => "pdte",
          password => "pdte_213456",
          port     => 5432,
          user     => "pdte_usr",
        },
}
[download]

and for the case that you have multiple keys which can also be missing in one of the hashes

  DB<132> $conf3 = { map { $_ => { %{$conf1->{$_}}, %{$conf2->{$_}} } 
+} keys %{{ %$conf1,  %$conf2 }} }
 => 
{
  db => {
          host     => "localhost",
          name     => "pdte",
          password => "pdte_213456",
          port     => 5432,
          user     => "pdte_usr",
        },
}
[download]

UPDATE

maybe easier to understand?

  DB<142> sub merge_hr {  return  { %{$_[0]}, %{$_[1]} } } 

  DB<143> $conf3 = { map { $_ => merge_hr( $conf1->{$_}, $conf2->{$_} 
+) } keys %{merge_hr($conf1,$conf2)} }
=> 
{
  db => {
          host     => "localhost",
          name     => "pdte",
          password => "pdte_213456",
          port     => 5432,
          user     => "pdte_usr",
        },
}
[download]