input lines:
2012/12/13 @ 13:32:35,585 @ ,, INFO  [EJB3Deployer] Starting java:comp multiplexer
2012/12/13 @ 13:32:34,585 @ ,, INFO  [EJB3Deployer] Starting java:comp multiplexer
2012/12/13 @ 12:32:35,485 @ ,, INFO  [EJB3Deployer] Starting java:comp multiplexer
2012/12/13 @ 13:35:35,585 @ ,, INFO  [EJB3Deployer] Starting java:comp multiplexer
2012/12/13 @ 14:32:35,585 @ ,, INFO  [EJB3Deployer] Starting java:comp multiplexer
2012/12/15 @ 13:32:35,612 @ ,, INFO  [EJB3Deployer] Starting java:comp multiplexer
2012/12/12 @ 11:32:35,585 @ ,, INFO  [EJB3Deployer] Starting java:comp multiplexer
2012/10/13 @ 13:32:45,735 @ ,, INFO  [EJB3Deployer] Starting java:comp multiplexer


 $\ = "\n" ; $, = "\t" ;
 use strict;

use warnings;

use Sort::External;


print "Script start time is\n ", scalar localtime();

open DATA, "sort_input.txt";
open OUTPUT, ">>sort_output.txt";

my $sortscheme = sub { my @flds_a = split(/,,/, $Sort::External::a);

                  my @flds_b = split(/,,/, $Sort::External::b);

                   $flds_a[0] cmp $flds_b[0]; };

#my $temp_directory = '/home/david/temp';

my $sortex = Sort::External->new(  mem_threshold => 1024**2 * 16,
								   sortsub         => $sortscheme,  
                                  #working_dir     => $temp_directory,
								  );

while (<DATA>) {   
 chomp;

    $sortex->feed($_);}

$sortex->finish;

while ( defined( $_ = $sortex->fetch ) ) 
{   
print OUTPUT $_;
}
 
 
 close DATA;
 close OUTPUT;
 
 print "Script end time is\n ", scalar localtime();