Depends on the languages you're familiar with... fn is common in some areas, _ is a popular "default variable", -> I see as an interesting addition although it would be equivalent to empty(_).
What I'm not sure about is @arr (why not @array?) and [] instead of {} for a map...
I'm not sure if it's beautiful, but it's definitely an interesting shortcut for common operations and I could immediately see why it's useful and already remember the syntax for it. I like it and the author seems to like it too, not everyone has to agree of course ;)
But we're talking syntax, not behavior. There's no reason they couldn't use {} to indicate maps and [] to indicate lists, even if they're the same data type. The point of this language is to be a cleaner syntax on top of PHP, so syntactical sugar like that is a Good Thing (TM).
uhm, beautiful