open(IN, $oneper) or die "Can't open file $oneper for reading: $!\n";
open(INDEX, ">:raw","$file.idx") or die "Can't open $file.idx for read/write: $!\n";

syswrite INDEX, pack('N',0),4;
while (<IN>) {
syswrite INDEX, pack('N', tell INDEX), 4;
}
close INDEX;