why not just enclose the sub-section of code in its own limited scope?

Simply for reason of aesthetics ;-)

As was already pointed out, there are many ways to skin the cat. I was just wondering whether Perl language really dictates us here to introduce a variable name, since in all other cases I can think of, an expression which is only needed once, doesn't need to be named. Having found this case where I could not do without introducing a variable, I considered it more likey that I was just missing some feature of Perl...