Here's a solution with just map and grep:

#!/usr/bin/env perl

use strict;
use warnings;

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

my @merged = 
    map { { %{$data1->[$_]}, %{$data2->[$_]} } }
    grep { $data1->[$_]{ACC} eq $data2->[$_]{ACC} } 0 .. $#$data1;

# Check results
use Data::Dump;
dd \@merged;
[download]

Output:

$ pm_1148951_merge_arrays.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,
  },
]
[download]