Like this?:

#!/usr/bin/perl
use strict;
use warnings;

use Data::Dump qw[ pp ];


my $data1 = [
    { 'NAME' => '1PAUL DY', 'DATE' => '12009-05-05', 'NUMBER' => '1000
+01', 'ACC' => '1A1A', },
    { 'NAME' => '2PAUL DY', 'DATE' => '2011-01-05', 'NUMBER' => '20033
+1', 'ACC' => '2A3B', },
    { 'NAME' => '4PAUL DY', 'DATE' => '42011-01-05', 'NUMBER' => '4200
+331', 'ACC' => '6A4B', },
];

my $data2 = [
  { 'EXT1' => '1b', 'EXT2'  => '12b', 'EXT3' => '13b', 'EXT4' => '14d'
+, 'ACC' => '1A1A', },
  { 'EXT1' => '2b', 'EXT2'  => '2b', 'EXT3' => '2b', 'EXT4' => '2d', '
+ACC' => '2A3B', },
  { 'EXT1X' => '4b', 'EXT2X'  => '4b', 'EXT3X' => '4b', 'EXT4X' => '4d
+', 'ACC' => '4A4B', },
];

for my $i ( 0 .. $#{ $data1 } ) {
    if( $data1->[ $i ]{ ACC } eq $data2->[ $i ]{ ACC } ) {
        $data1->[ $i ]{ $_ } = $data2->[ $i ]{ $_ } for keys %{ $data2
+->[ $i ] }
    }
}

pp $data1;

__END__
C:\test>1148951.pl
[
  {
    ACC    => "1A1A",
    DATE   => 12009-05-05,
    EXT1   => "1b",
    EXT2   => "12b",
    EXT3   => "13b",
    EXT4   => "14d",
    NAME   => "1PAUL DY",
    NUMBER => 100001,
  },
  {
    ACC    => "2A3B",
    DATE   => 2011-01-05,
    EXT1   => "2b",
    EXT2   => "2b",
    EXT3   => "2b",
    EXT4   => "2d",
    NAME   => "2PAUL DY",
    NUMBER => 200331,
  },
  { ACC => "6A4B", DATE => 42011-01-05, NAME => "4PAUL DY", NUMBER => 
+4200331 },
]
[download]