The best thing to debug your problem is "perl debugger" (perl -d) and the simplest thing is "print". Did you check whether $_ or $fields[5] contain any value? I found a problem and a suspicion. Here are following.

1. You do not have a space between split and its argument
2. There could be good chances that while copy/pasting the text you unintentionally converted the tabs into spaces and everything goes into $fields[0]

I hope this helps