[This post notes differences between the fifth and sixth editions.]
With the added emphasis on Unicode, we had to update the chapter on input and output a bit. If we are going to talk about Unicode, we need to talk about encodings, which expands the material on three argument open and brings in binmode too.
For this update, we also introduced filehandle references, although we did not call them references, really. We still present most of the chapter using bareword filehandles, and once we have covered everything we show how you can do the same things with filehandles in variables. However, we still save the meat of filehandle referencs to Intermediate Perl.
Some people think everyone should be using lexical filehandles all of the time, but even if you want to do that for new code, you still have to understand what people did in old code so we have to cover the legacy syntax (and some people would even object to calling it “legacy”).