Couldn't you just use shared variables? Put the XML is one shared scalar string, and have another shared variable which is constantly tested in the threads, which would tell it to reload the XML from the shared scalar or hash?

A signal might work to trigger the reload, but as Corion pointed out, signals and threads are quite tricky to get to work right, see Threads, bash, and networking for example; whearas constantly checking a shared variable is quite reliable.

Another option is to use the fact that you can share filehandles with threads thru the fileno. See FileHandles and threads. You may be able to place your XML in the main thread's defined filehandle, and have the threads re-read it on some command... either thru shared var or a signal of some sort.