for my $key1 (keys %$returnedData) {
   for my $key2 (keys %{$returnedData->{$key1}}) {
       print "$key1.$key2 = $returnedData->{$key1}->{$key2}\n";
   }
}

##</code><code>##

C:\tmp>type erick.pl

my $returnedData = {
   key1 => {
      key5 => 'data1',
      key6 => 'data2',
      key7 => 'data3',
      key8 => 'data4',
   },
   key2 => {
      key9 => 'data5',
      key10 => 'data6',
      key11 => 'data7',
      key12 => 'data8',
   },
   key3 => {
      key13 => 'data9',
      key14 => 'data10',
      key15 => 'data11',
      key16 => 'data12',
   },
   key4 => {
      key17 => 'data13',
      key18 => 'data14',
      key19 => 'data15',
      key20 => 'data16',
   },
};

for my $key1 (keys %$returnedData) {
   for my $key2 (keys %{$returnedData->{$key1}}) {
       print "$key1.$key2 = $returnedData->{$key1}->{$key2}\n";
   }
}

C:\tmp>erick.pl
key2.key10 = data6
key2.key12 = data8
key2.key9 = data5
key2.key11 = data7
key4.key19 = data15
key4.key18 = data14
key4.key17 = data13
key4.key20 = data16
key1.key5 = data1
key1.key6 = data2
key1.key8 = data4
key1.key7 = data3
key3.key15 = data11
key3.key13 = data9
key3.key14 = data10
key3.key16 = data12