Improving performance of checksum calculation

[Crackers2]

I have the following code to calculate a checksum (the checksum algoritm is not under my control):

  open my $fh, "<", "myfile" or die;
  my $check_value = 0;

  while (1) {
    my $buf;

    my $cnt = read $fh, $buf, 4;
    if (!defined $cnt) {
      die("Error while reading from input file: $!");
    } elsif ($cnt==0) {
      last;
    } elsif ($cnt%4) {
      die("File or chunk does not seem to have a length that's a multi
+ple of 4 bytes");
    }

    my $val = unpack("L",$buf);

    $check_value ^= $val;
    my $rotate = $check_value & 0x80000000;
    $check_value = ($check_value << 1 ) & 0xffffffff;
    if ($rotate) { $check_value |= 0x01; }
  }

It works but is terribly slow. Any suggestions to speed it up?. This looks like the kind of thing that would be a lot faster in C, and trivially so in assembler because of the nice rotate instructions; I'd rather avoid shelling out to a subprocess just for this though.