As it has been mentioned, this isn't a new idiom at all. We have forever been saying open $fh, $mode, $file or die "Can't open $file: $!", and of course, that same approach generalizes to any kind of error handling.

This is one of the facets of Perl's adherence to linguistic principles. (I think Larry mentions that in his essay about the subject.) It tries to let the programmer arrange expressions in whichever way he deems necessary to stress the really important part. That's why we have the statement modifiers, for example. next if $long_and_complex_conditional_here is a familiar and beloved sight in Perl code; I often cringe when I'm in C world and have to arrange the same thing the other way around.

That is no particular reason to hate unless, of course. if vs unless is just another of these facets: it lets you build expectations appropriately. I use unless when I expect that the condition will only rarely be true. For me, if not is an indication that the condition will tend to go either way, or even be true more often than not — the opposite of the same thing expressed with unless.