#! perl -slw
use strict;

my( $read, $buf, $ch, $pair );
my $offset =  0;

open (FH, '< :raw', $ARGV[ 0 ]) or die $!;

while ( $read = sysread( FH, $buf, 4096 - $offset, $offset ) ) {
	for my $p ( 500 .. $read ) {
		$pair = substr( $buf, $p - 500, 1 ) . substr( $buf, $p, 1 );
	}
	$buf = substr( $buf, $read - 500, 500 );
	$offset = 500;
}

close FH;