for (my($k,$v)=each %{$data2->[0]}){
  $_->{$k} = $v for @$data1;
}
[download]

That works for a single key/value element in $data2->[0], but given that it can work for only a single element, why not use the ocularly even easier

my ($k,$v) = %{$data2->[0]};

$_->{$k} = $v for @$data1;
[download]

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', 'X' => 'Y', } ];

for (my($k,$v)=each %{$data2->[0]}){
  $_->{$k} = $v for @$data1;
}

print Dumper $data1;

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

for (my($k,$v)=each %{$data2->[0]}){

You may need to use while in place of for to get all the value in %{$data2->[0]}