my ($in) = <CURR>;

Is that it puts <CURR> in list context. That causes it read all the lines in your file and then put the first one in $in! Instead you want:

my $in = <CURR>;

Which will read just one line. Reading from two files should work with something like:

  open FILE1, "<", "file1.txt" or die $!;
  open FILE2, "<", "file1.txt" or die $!;
  while (!eof(FILE1) and !eof(FILE2)) {
    my $line1 = <FILE1>;
    my $line2 = <FILE2>;
    # ...
  }
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-sam