#! perl -slw
use strict;

our $NBUF //= 5000;
our $IBUF //= 2e6;

my $start  = time;

my @outFHs;
my @outBufs;
my $n = 0;
my( $o, $buf ) = 0;

open DISK, '<', $ARGV[0] or die $!;

while( read( DISK, $buf, $IBUF, $o ) ) {
    open RAM, '<', \$buf;
    while( my $line = <RAM> ) {
        unless( $line =~ /\n$/ ) {
            $buf = $line;
            $o = length $buf;
            next;
        }
        ++$n;
        my $key = substr( $line, 7, 3 ) % 600;
        if(
            push( @{ $outBufs[ $key ] }, $line ) > $NBUF
        or
            !$outFHs[ $key ] && @{ $outBufs[ $key ] } > rand( $NBUF )

        ) {
            unless( defined $outFHs[ $key ] ) {
                open $outFHs[ $key ], '>', "$key.out" or die $!;
            }
            print { $outFHs[ $key ] } @{ $outBufs[ $key ] };
            @{ $outBufs[ $key ] } = ();
        }
    }
}

print { $outFHs[ $_ ] } @{ $outBufs[ $_ ] } for 0 .. $#outBufs;
close $_ for @outFHs;
close DISK;

printf "Took %d seconds for $n records\n", time() - $start, $n;

__END__
C:\test>Ibufd.pl -IBUF=20971520 -NBUF=6000 1GB.csv
Took 114 seconds for 16777216 records