One way (update: see the Perl Data Structures Cookbook):

c:\@Work\Perl\monks>perl
use strict;
use warnings;

use Data::Dumper;

my $data1 = [
{
   'NAME' => 'PAUL DY',
   'DATE' => '2009-05-05',
   'NUMBER' => '00001',

 },
   {
   'NAME' => 'ANTHONY RD',
   'DATE' => '2012-01-07',
   'NUMBER' => '00003',
 },
  {
   'NAME' => 'RUTH RD',
   'DATE' => '2018-01-07',
   'NUMBER' => '00023',
 },

 ];

my $data2 = [
  {
    'CODE' => 'X11',
  }
];

foreach my $hashref (@$data1) {
  %$hashref = (%$hashref, %{ $data2->[0] });
  }

print Dumper $data1;

__END__
$VAR1 = [
          {
            'NAME' => 'PAUL DY',
            'DATE' => '2009-05-05',
            'CODE' => 'X11',
            'NUMBER' => '00001'
          },
          {
            'NAME' => 'ANTHONY RD',
            'DATE' => '2012-01-07',
            'CODE' => 'X11',
            'NUMBER' => '00003'
          },
          {
            'NAME' => 'RUTH RD',
            'DATE' => '2018-01-07',
            'CODE' => 'X11',
            'NUMBER' => '00023'
          }
        ];
[download]

Update 1: Note that using this method, if there is a key in the hash referent of the single element of the  $data2 array referent that is the same as one in a hash referent of any  $data1 array referent, the value of the former will silently overwrite the value of the latter. (Update: For instance, see what happens if  $data2 happens to be
    my $data2 = [ { 'CODE' => 'X11',  'NAME' => 'JONES', } ];
instead. (Update: This exact problem is discussed more clearly in the FAQ referenced by BillKSmith here.))

Update 2: Here's a testing framework for playing around with other approaches. See Test::More, Test::NoWarnings.

perldoc -q "How do I merge two hashes?"
[download]