sub BLOCKLENGTH () { 1 << 12 }; # TIMTOWTDI =)

$_ = '';
while(sysread STDIN, $_, BLOCKLENGTH, length){
     s///g; # you know
     syswrite STDOUT, substr($_, 0, -BLOCKLENGTH, ''); # the fourth argument to substr will replace 0 .. -BLOCKLENGTH
};

syswrite STDOUT, $_;