my $search = sprintf("%03d",20); # pad the search thingy
my $ind_length = 3;    # length of sorted index column
my $line_length = 8;   # line length
my $size = (-s $file);
die "Corrupt file" unless $size % $line_length == 0;
open(IND,$file) || die $!;  # get the file ready
my $min = 0;        # lower bound
my $max = $size;    # upper bound

my $result = undef;
my $match;
while(1){
  my $pos = $ind_length * int(
       ($min/$ind_length + $max/$ind_length) / 2
         ); # find the middle (make sure it is at the
            # beginning of a row
  last if $pos == $min;   # there is no result
  seek(IND,$pos,0);       # go to the middle
  sysread(IND,$match,$ind_length); # read the info

  # found it!
  if( $match eq $search ){
    seek(IND,($pos+$ind_length+1),0);
    sysread(IND,$result,$ind_length); # read the real index
    last;
  }
  if( $match gt $search ){
    $max = $pos;
  }else{
    $min = $pos;
  }
}
if( defined $result ){
  print "Found it ($result)\n";
}