The single-letter variable was for the purposes of the example. The example still holds with a more descriptive name--is $results_file a filehandle or a string containing the name of a file?
More knowledge about the ambiguities of Perl is always a good thing. I don't see this practice as a replacement for knowing how things work. It just adds clarity. Braces are a nice visual cue that make a filehandle variable instantly recognizable as a filehandle.
When's the last time you used duct tape on a duct? --Larry Wall