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}";
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Prints:

200.00
[download]