open IN, "<", $textfile or die "Can't open textfile: $!";
binmode IN;
open IDX, ">", "$textfile.idx" or die "Can't create index file: $!";
binmode IDX;
print IDX pack "N", 0;
while(<IN>) {
    print IDX, pack "N", tell IN;
}

##</code><code>##

open TEXT, "<", $textfile or die "Can't open textfile: $!";
open IDX, "<", "$textfile.idx" or die "Can't open index file: $!";
binmode IDX;
seek IDX, 4*$lineno, 0;
read IDX, my($buf), 4;
seek TEXT, unpack("N", $buf), 0;
$line = <TEXT>;