This is qrpff.pl. It decodes CSS, which is the Content-Scrambling System used to encrypt video data on DVDs:

$_='while(read+STDIN,$_,2048){$a=29;$c=142;if((@a=unx"C*",$_)[20]&48){
+$h=5;
$_=unxb24,join"",@b=map{xB8,unxb8,chr($_^$a[--$h+84])}@ARGV;s/...$/1$&
+/;$d=
unxV,xb25,$_;$b=73;$e=256|(ord$b[4])<<9|ord$b[3];$d=$d>>8^($f=($t=255)
+&($d
>>12^$d>>4^$d^$d/8))<<17,$e=$e>>8^($t&($g=($q=$e>>14&7^$e)^$q*8^$q<<6)
+)<<9
,$_=(map{$_%16or$t^=$c^=($m=(11,10,116,100,11,122,20,100)[$_/16%8])&11
+0;$t
^=(72,@z=(64,72,$a^=12*($_%16-2?0:$m&17)),$b^=$_%64?12:0,@z)[$_%8]}(16
+..271))
[$_]^(($h>>=8)+=$f+(~$g&$t))for@a[128..$#a]}print+x"C*",@a}';s/x/pack+
+/g;eval
[download]

I did not write this; it was written by Keith Winstein and Marc Horowitz at MIT.

Some time ago I was hired by Wired magazine to explain how this program worked. I promised you folks that when the magazine got to the newsstand, I would put my original, unedited version online.

Here it is.

--
Mark Dominus
Perl Paraphernalia