Heh. One way it could be easier is if there was a reliable way to know if a given Perl module is thread-safe! Most pure Perl code will be but XS modules often aren't unless someone has gone to the trouble of making them that way.
The same can be said of forking of course, you could say that DBD::mysql isn't fork-safe and you'd be kind of right. But there's definitely fewer problems with forking and XS code.
Also, the performance of threads, particularly for smallish tasks, is really quite bad. I know you're going to ask me to quantify that statement but I really don't have the time. I've seen it benchmarked plenty of times before though, so you can probably find a fork versus threads benchmark around.