The other day one of the other developers here was looking at some of my code where I'd done something like:

if (-d $foo) {
    ...
}
elsif (-s _) {
    ...
}
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He had never seen the _ file-test optimization and leaped to the conclusion that it was a syntax error. My response to him was not very kind--I literally told him to RTFM--but I can sort of sympathize with the position he was in. For whatever reason he was unaware of that particular feature, and it may not be immediately obvious where to find out about it. In hindsight, sure, go to the file-test-operator documentation, but it could just as easily have been some obscure variable/bareword syntax, or a call to a function named '_'. And for that matter, if you didn't know the right perldoc command to show the file-test documentation, it'd probably take a few tries to find it. I've been using perl since version 3.something, but there are lots of times I get tired of having to keep track of all the syntax idiosyncracies.