Perl bootblock checksummer

sum-bootblock.pl

#! /usr/bin/env perl -w

use strict 'subs';
use bytes;

sub checksum(\$) {
	my $data = shift;
	my $sum = 0;
	for (my $i = 0; $i < 1024; $i += 4) {
		$sum += unpack('N', substr($$data, $i, 4));
		if ($sum > 0xffffffff) {
			$sum = ($sum + 1) & 0xffffffff;
		}
	}
	return (~$sum) & 0xffffffff;
}

die "need two arguments" unless $ARGV[0] and $ARGV[1];

my $data;

open(INPUT, '<:raw', $ARGV[0]) or die "can't open $ARGV[0]\n";
read(INPUT, $data, 1024);
close(INPUT);

my $diff = (1024 - bytes::length($data));
$data .= "\0" x $diff unless $diff == 0;
substr($data, 4, 4) = pack('N', checksum($data));

open(OUTPUT, '>:raw', $ARGV[1]) or die "can't open $ARGV[1] for writing\n";
print OUTPUT $data;
close(OUTPUT);