Why would $hdr be empty? Two possibilities. First, it's a blank line. chomping it will remove the newline, leaving a possibly empty string. The second possibility is that the open failed. Here's a better error line:

open(FILE, $data) or die "Can't open ($data): $!";

That gives you the name of the file, surrounded by non-whitespace characters as a sanity check, along with the system error message. You'll have better luck if you let Perl help you.

Update: If it's not a file, don't open it. open works on files (in the Unix sense). If you have the data you want to munge already in an array or a scalar, use split or a regex to get at it. That's the problem.