I more or less agree with a rule of thumb for $_ in Perl. If you have to use $_ explicitly, it might be better to use something else. I make exceptions for map and grep, though. I pretty much always expect $_ to appear there. In this specific case, I don't think it hurts anything to stick in an explicit $_ (but I'd probably like it better without). You could achieve that, if you really want, by reversing the test: grep 10_000 > -s, @ARGV.

All just my opinions, of course.