In general I try very hard to avoid the race condition that come from assuming the result of a filetest operator remains valid. In this case it is possible that something creates the directory between the moment of the -d and the mkdir and that's a case I'd prefer to work instead of fail. In many real cases this is impossible or not an issue of course, but I still prefer my code to be able to handle such cases. So normally I'd write:

if (!mkdir($dir)) {
   my $err = $!;
   die "Could not create $dir: $err" unless -d $dir;
}
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