The solutions the others posted are great, but they didn't explain why, or at least not clearly. The problem is that you're using the same file handle (FH) for all of your files, so when you try to close two files, you end up closing the same one twice.

btw, another way of doing it is to use the FileHandle module:
my $fh = FileHandle->new('filename', 'r') or die(...);

There is another problem with your code. When you have an error, you're returning the return value of warn, whatever that is. You should probably either die, or return undef.