Yes, once sorted, the algorithm just reads the file once in a linear fashion. So this should be great for your humongous file.

The warning message should give you a line number in the code and often you also get the line number of the input file. One common way to get an uninitialized value is when there is a blank line in the file - this causes the split to fail (no results). An extra carriage return is easy to get missed since they are "invisible". I often put: next if /^\s*$/; which will go to the next input line if the current line contains nothing but white spaces.

I think that I already mentioned that normally you probably should be splitting on the regex /\s+/ which is the default. white space (\s) includes all of the following: the space of course,\n\r\f\t any contiguous sequence of those gets removed. Splitting on just tab characters (\t) can cause problems if there are sometimes extra space characters in there that you cannot see with the editor.

I think you are on the right track - keep at it!