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

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