Well,
as I wrote, I was playing around with local, to perform a
kind of nothing-left-open-test - and nothing doesn't work.
What is interessting is, that there is now an example at
http://www.cpanforum.com/dist/threads that works,
it causes the threads to break their sleep, and the signals SIGUSR1
were set or thrown by mum.

As soon as I changed it a bit (code even there) that signals from
'outside' should be caught - it fails again: the SIGUSR1 were sent
to the threads, mum waits,but the thread's sleep is not broken.

So it seems this is something that Perl can't do!

Surprised and and bit disappointed,
Carl