open (INFILE, $infile) || die "but... $!";
open (OUTFILE, ">$outfile") || die "but... $!";

my @hold;
while(<INFILE>){
    if ($. < 12){
        push @hold, $_;
        next;
    }
    if ($. == 12){
        # grab some data from the 12th line
        # perform some funky manipulation on it
        print OUTFILE "new funky data from 12th line";
        print OUTFILE @hold;
        next;
    }
    print OUTFILE $_;
}