open (FIN, "<$ARGV[0]") || die "Input file: $!\n";
open (FOUT, ">$ARGV[0]-randomized") || die "Output file: $!\n";
$flag = 1;

#set autoflush on output filehandle
$oldfh = select(FOUT);
$| = 1;
select ($oldfh);

while (<FIN>) {
    if ($flag) {
        seek (FOUT, 0, 0) || die "Seek to beginning: $!\n";
        $flag = 0;
    } else {
        seek (FOUT, 0, 2) || die "Seek to end: $!\n";
        $flag = 1;
    }

    print FOUT $_;
}

close (FOUT);
close (FIN);