It would be much better for the Perl program that generates the data to write it to a database. However, going with what you have been given:

use warnings;
use strict;
use Data::Dump::Streamer;

my $data = <<DATA;
'main_data' => [
    {
        'LEVEL' => '12',
        'CARNUM' => '1',
        'REGIS#' => '1233456678987645RD    ',
        'ACCOUNT' => '6510334',
        'YEAR' => '2008'
    },
    {
        'AMOUNT' => '200.00'
    },
    {
        'LEVEL' => '0',
        'CARNUM' => '2',
        'REGIS#' => '2234565478345R56T    ',
        'ACCOUNT' => '6510334',
        'YEAR' => '2008'
    },
    {
        'AMOUNT' => '0.00'
    }
    ],
DATA

local $\;

open my $InFile, '<', \$data;
my $str = <$InFile>;
close $InFile;

no strict "vars";
eval "\%hash = ($data);";

my %accounts;

while (@{$hash{main_data}}) {
    my ($accountInfo, $amount) = splice @{$hash{main_data}}, 0, 2;
    my $acct = $accountInfo->{ACCOUNT};
    my $reg =  $accountInfo->{'REGIS#'};
    
    delete @{$accountInfo}{'ACCOUNT', 'REGIS#'};
    $accounts{$acct}{$reg} = $accountInfo;
    $accounts{$acct}{$reg}{AMOUNT} = $amount->{AMOUNT};
}
use strict;

print "$accounts{6510334}{'1233456678987645RD    '}{AMOUNT}";
[download]

Prints:

200.00
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Would be possible to add the amount 'AMOUNT' => '200.00' to the first hash right after the 'LEVEL' => '12', because these values belong to the information above them, and they are separated making things more difficult.

If I understand the meaning of your question correctly, then the answer is no. It is certainly possible to add anything you want to any of the hashes, but, if you mean by “right after” that there should be some ordering, then you can't achieve that. If you just want it to appear in the same hash, and don't care (as you mustn't) about ordering, then (as moritz pointed out when you asked the same question before) why not just do what you're suggesting and test the result yourself?

Try:

  # Assuming your outside hash is named %h,
  print $h{main_data}[1]{AMOUNT} . "\n";
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for (map {@$_} values %h){
  print "Account " . $_->{ACCOUNT} . " AMOUNT: " if exists $_->{ACCOUN
+T};
  next unless exists  $_->{AMOUNT};
  print   $_->{AMOUNT} . "\n";
}
# --- Output ---
# Account 6510334 AMOUNT: 200.00
# Account 6510334 AMOUNT: 0.00
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AOH not HOAOH

If you are talking about the content inside the [..], then I'd agree that it contains an AOH.

From my perspective, the outermost
'main_data' => [...
indicates that it is either a hash or hashref. Of course, the programmer could be misusing the "=>" instead of a comma, making the outermost element an array, but that is marginally perverse.

So. in the absence of information to the contrary, I'll stick with my interpretation of HOAOH.

     Have you been high today? I see the nuns are gay! My brother yelled to me...I love you inside Ed - Benny Lava, by Buffalax

my %hash = (
    'main_data' => [ 
                              { 
                                'LEVEL' => '12', 
                                'CARNUM' => '1', 
                                'REGIS#' => '1233456678987645RD    ', 
                                'ACCOUNT' => '6510334', 
                                'YEAR' => '2008' 
                              }, 
                              { 
                                'AMOUNT' => '200.00' 
                              }, 
                              { 
                                'LEVEL' => '0', 
                                'CARNUM' => '2', 
                                'REGIS#' => '2234565478345R56T    ', 
                                'ACCOUNT' => '6510334', 
                                'YEAR' => '2008' 
                              }, 
                              { 
                                'AMOUNT' => '0.00' 
                              }
]);

my @amounts = map { $_->{AMOUNT} } @{$hash{main_data}};
print "@amounts\n";
[download]

Would be possible to add the amount 'AMOUNT' => '200.00' to the first hash right after the 'LEVEL' => '12', because these values belong to the information above them, and they are separated making things more difficult.

Sure it's possible. What have you tried so far? where have you had problems?