open(IDX, "< index") or die;
open(DAT, "< datafile") or die;
# assume $lnum is the line number to go to
seek(IDX, 8*$lnum, 0) or die;
read(IDX, $offset, 8) or die;
seek(DAT, $offset, 0) or die;
# now DAT filehandle is positioned to read the
# record you wanted