You may be interested in this implementation of the Adler-32 checksum algorithm (see Adler-32).

use warnings;
use strict;

use constant MOD_ADLER => 65521;

print Adler32('abc def ghi') . "\n";
print Adler32('acb def ghi') . "\n";

sub Adler32 {
    my $data = shift;
    my $scan = 0;
    my $len = length $data;
    my $a = 1;
    my $b = 0;

    while ($len) {
        my $tlen = $len > 5550 ? 5550 : $len;
        
        $len -= $tlen;
        do {
            $a += ord (substr $data, $scan++, 1);
            $b += $a;
        } while (--$tlen);
        
        $a = ($a & 0xffff) + ($a >> 16) * (65536 - MOD_ADLER);
        $b = ($b & 0xffff) + ($b >> 16) * (65536 - MOD_ADLER);
    }

    $a -= MOD_ADLER if $a >= MOD_ADLER;
    $b = ($b & 0xffff) + ($b >> 16) * (65536 - MOD_ADLER);
    $b -= MOD_ADLER if $b >= MOD_ADLER;

    return ($b << 16) | $a;
}
[download]

Prints:

378209230
378274766
[download]

Note that this is a translitteration from the C code - $a and $b would be better renamed. :)