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open(IDX, "> index") or die;
open(DAT, "< datafile") or die;
...
} while (defined $line);
close(IDX);
close(DAT);

open(IDX, "< index") or die;
open(DAT, "< datafile") or die;
...
seek(DAT, $offset, 0) or die;
# now DAT filehandle is positioned to read the
# record you wanted