my $first_line_indicator=1;
my $outfile_counter=0;
while (my $line = <FILEin>){
  if ($first_line_indicator==1){
    $outfile_counter++;

    # open new write destination
    # write $line;

    $first_line_indicator=0;
  }else{
    # write $line;
    #close write destination

    $first_line_indicator=1;
  }
}