Assuming pp_select is what implements select (big assumption, but that looks like what this code does) I think the select operation probably is threadsafe.

The default output location looks like it's PL_defoutgv, which is a #define for a component of aTHX, which I _believe_ is thread local.

Since pp_print also uses the same output dest, I think his whole script will work as expected, assuming the sleep "sleeps" the thread and not the process...
--
Mike

Edit: Added caveat about sleep