Here's another way to see -e isn't a function:
I personally believe that it's too bad it isn't. Actually, it even seems one, not to mention that its documentation is available under perldoc -f -X! Of course Perl 6 knows better by making all operators actual functions. But I wouldn't dare asking that much in 5's realms: 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 _, but at this point ideally those tests may be overridden and without interfering at all with the additional magic, which would be completely orthogonal.