In addition to what Corion said about using an event loop, Glib( base of Gtk2) has a neat way to watch filehandles, see Roll your own Event-loop. The "man Glib::MainLoop" manpage does not mention anything about not working on win32, I think I will ask on the Perl/Gtk2 maillist.

UPDATE: Answers from the Perl/Gtk2 maillist indicate that the Window's port of Glib does include some "select" functionality, so Glib::IO->add_watch() works on Win32, allowing you to read the output from piped-opened filehandles in a callback. This is great news. :-)