In Anne McCaffrey's Pern books, the threads are spores of fungus, silvery in color, that grow in the Pern's neighboring planet Red Star, travel across the intervening space when the planets are close each other (this time is known as Pass) and drop on Pern's surface. They burrow to the ground and eat everything organic they get in touch with. Those things seem to prefer places with vegetation.

There are many methods with which the Pernfolk fight the threads. The oldest method involves Pern's dragons that can scorch the threads mid-air. Other methods, in case the threads reach the ground, include (as far as I can remember) use of acids and flamethrowers. There's also a type of insect that eats the thread (or so says Zerotime; I have some faint memories about this, I have to check this.)

(The Perlese dragons fight the Thread module. The authors of the Camel Book do note that "Pern" is what you get when you do "Perl"++ twice.)

The Thread module in Perl implements threads. As of Perl 5.6, this module is deprecated; It deals with the "5005threads", the old thread model from Perl 5.005 days (that is not that good, for example, many special variables were not thread-safe!). Code for 5.6 and later should use the threads and threads::shared modules (note lower case), which use "ithread" model (separate thread for each intepreter). For ithread users, the Thread module attempts to emulate 5005threads, but it's probably better to stick with threads from now on. To see which thread models are supported on your installation, see perl -V and look for "use5005threads" or "useithreads".