use Data::Dumper;  # So that we can inspect our result.

# Initial state.
my $aref = [
  {
    'xml_api_reply' => {
      '-version'       => '1',
      'weather'        => {
        '-row'            => '0',
        'current_conditions' => { }
      },
    },
  },
];

# Now add 'report_date' key and its value at the appropriate level.
$aref->[0]{'xml_api_reply'}{'weather'}{'report_date'} = { '-data' => '08-19-2012 @ 15:40:26' };

# Inspect the result.
print Dumper $aref;