use warnings;
use strict;
use Data::Dumper;

$Data::Dumper::Sortkeys=1;

my $data = {
          'DATE' => '2013-04-25',
          'TW' => '1',
          'REF' => '1234567',
          'ACC' => '33456790'
        };

$data = {%$data, DOC => 'EMAIL'};

print Dumper($data);

##</code><code>##

use warnings;
use strict;
use Data::Dumper;

$Data::Dumper::Sortkeys=1;

my $data = {
          'DATE' => '2013-04-25',
          'TW' => '1',
          'REF' => '1234567',
          'ACC' => '33456790'
        };
my $new = {
          DOC => 'EMAIL' ## NOTE: no need for quotes on thing
                         ##       to the left of `=>'
        };

$data = {%$data, %$new};

print Dumper($data);