use strict;
use warnings;
use Fcntl ':seek';
use bigint;

use constant CHUNK => 65536;

sub calc {
    my $file  = shift or die "no filename given\n";
    my $hash  = -s $file;
    my $chunk = CHUNK;

    $hash < $chunk and die "$file is too small ($hash bytes < $chunk)\n";

    open my $in, '<', $file or die "Cannot open $file for input: $!\n";
    local $/ = \$chunk;

    my @val = unpack 'L*' , readline($in);
    for (my $j = 0; $j < $#val; $j += 2) {
        $hash += ($val[$j] << 32) + $val[$j+1];
        $hash &= 2 ** 64 - 1;
    }
    seek($in, SEEK_END, -$chunk);
    @val = unpack 'L*' , readline($in);
    for (my $j = 0; $j < $#val; $j += 2) {
        $hash += ($val[$j] << 32) + $val[$j+1];
        $hash &= 2 ** 64 - 1;
    }

    close $in;
    return sprintf '%016x', $hash;
}

open my $out, '>', 'test.deleteme';
print $out ' ' x (65536*4);
close $out;

print calc('test.deleteme'), $/;