open IN, "bigfile.txt";
open  IDX, ">bigfile.idx";
binmode IDX;
print IDX, pack "N", 0;
while(<IN>) {
    print IDX pack "N", tell IN;
}

##</code><code>##

my $lineno = 10000;

open IN, "bigfile.txt";
open IDX, "bigfile.idx";
binmode IDX;
seek IDX, 4 * $lineno, 0;
sysread IDX, (my $buffer), 4;
seek IN, unpack("N", $buffer), 0;
my $line = <IN>;