I think one of the reasons -f isn't a function is that it violates the naming rules. Identifier names have to start with letters, and dashes are not allowed. -f violates that. Changing it would probably mean changing the lexer. Which I don't think many people are willing to do.

perhaps one difficulty in this sense is the new stacking facility à la -f -w -x $file however AIUI from the docs that's just syntax fancy. Which I read in the sense that the parser takes care of it making it into the equivalent of -x $file && -w _ && -f _,

I was under the impression that the stackability was archieved by -f and friends returning magical values. But if there's magic, if doesn't show with Devel::Peek::Dump, so I'm probably wrong in my impression.