my $filepath = "OP.txt";
my $newdata = readdt2($filepath);

sub readdt2 {
  my $ifn = shift; # how does this work
  open(my $IFH, "<$ifn") or die "cannot open file $ifn\n";
  my $line;
  my @nucleotides = ("A","C","G","T");
  my %ret; #why did he name a hash return?? that really confused me
  my @tmp;
  for(my $j=0;$j<4;$j++) {
    $ret{$nucleotides[$j]} = []; #dont understand 
    $line = <$IFH>;
    chomp($line);
    @tmp = split(/\s+/,$line);
    for(my $i=0;$i<=$#tmp;$i++) {
      $ret{$nucleotides[$j]}->[$i]= $tmp[$i] ;  #dont understand 
    }
  }
  close($IFH);
  return(\%ret);
}