Maybe that a hash can help you :

#!/usr/bin/perl -w
use strict;
use Data::Dumper;

my %buf;
while( my $line = <DATA> ){
    next unless $line;
    chomp $line;
    $line =~ s/,\s*$//;
    my $accnum = (split ',', $line)[3];
    $buf{$accnum} = $line if $accnum; 
}

print Dumper \%buf;

__DATA__
20080618,adfadf,adfadf,abc123,ljlkjll,
20080618,jfjfjf,bmbmbm,abc123,zzzdddd,  

20080618,343434,959595,abc123,fjadfaf, 
20080618,adfadf,adfadf,xyz123,ljlkjll,
20080618,adxxxf,gggsss,xyz123,ggaaddf, 
20080618,dddfff,dfdfdd,xyz123,xxxxxd,
[download]

Output : 

$VAR1 = {
          'xyz123' =>'20080618,dddfff,dfdfdd,xyz123,xxxxxd',
          'abc123' =>'20080618,343434,959595,abc123,fjadfaf'
        };
[download]

hth,
PooLpi