I use a fairly simple method, for some files which I use as indices. It seems fairly efficient. I open the bit file, read in chunks 64K at a time, and use some simple operations to determine 'on' bits.

I only care about bytes that have at least one 'on' bit, so this makes things a little easier.

Here is some code:

while (sysread(INDEX, $index_data, 65536)) {
  # so zip round it until we hit a byte with any bit
  # set to one
 for ($z = 0 ; $z < length ($index_data) ; $z++ ) {
    next unless ord(substr($index_data,$z,1));
    # so this byte contains something
    foreach (0..7) {
      if ( (2 ** $_) & ord(substr($index_data,$z,1)) ) {
        # this bit is ON
        # do stuff here
      }
    }
  }
}
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BTW, I had a bad bug with this code initially. Instead of:

    next unless ord(substr($index_data,$z,1));
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I had:

    next unless substr($index_data,$z,1);
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I leave it to the reader to discover the evil bug in this code :-)