sub quickMD5 {
    my $fh = shift;
    my $md5 = new Digest::MD5->new;

    $md5->add( -s $fh );

    my $pos = 0;
    until( eof $fh ) {
        seek $fh, $pos, 0;
        read( $fh, my $block, 4096 ) or last;
        $md5->add( $block );
        $pos += 1024**2;
    }
    return $md5;
}

##</code><code>##

#! perl -slw
use strict;
use Time::HiRes qw[ time ];
use Digest::MD5;

sub quickMD5 {
    my $fh = shift;
    my $md5 = new Digest::MD5->new;

    $md5->add( -s $fh );

    my $pos = 0;
    until( eof $fh ) {
        seek $fh, $pos, 0;
        read( $fh, my $block, 4096 ) or last;
        $md5->add( $block );
        $pos += 1024**2;
    }
    return $md5;
}

open FH, '<', $ARGV[0] or die $!;
printf "Processing $ARGV[0] : %u bytes\n", -s FH;

my $start = time;
my $qmd5 = quickMD5( *FH );
printf "Partial MD5 took %.6f seconds\n", time() - $start;
print "Partial MD5: ", $qmd5->hexdigest;

##</code><code>##

C:\test>md5t 500MB.csv
Processing 500MB.csv : 536870913 bytes
Full MD5 took 6.350180 seconds
Full MD5: 3c81ccb7d2d7febc96c92b4d7dd4c797

C:\test>quickMD5 25GB.csv
Processing 25GB.csv : 26843545600 bytes
Partial MD5 took 88.150608 seconds
Partial MD5: 408ee1dabed25f1fbe4b25511c8b8287

C:\test>quickMD5 500MB.csv
Processing 500MB.csv : 536870913 bytes
Partial MD5 took 2.081898 seconds
Partial MD5: 894c2792caeaac64072d7189d5724ecc

C:\test>md5t 25GB.csv
Processing 25GB.csv : 26843545600 bytes
Full MD5 took 302.419120 seconds
Full MD5: 24ce5b913f2f49876f0f24031b9b5d9b