At first glance, nothing significant is standing out to me. I mean, I'd not pass in the \%threads reference to forkportcheck, but handle that in the caller, and I'm not seeing why you have the inner while, unless it's to force the output in the same order as the input (which seems odd), but those are relatively minor.
The only thing I can think of that would improve this is to use event-based sockets. Which would be much easier if you were using AnyEvent, Coro, POE, or one of a myriad of other event systems available for Perl. This would eliminate the fork, allowing it all to be done in a single process. If you're connecting to systems on the internet, I doubt this will be much of a gain, and even if it's all on your intranet, it may be too little of a gain to be noticeable (though only one way to be sure!).