C> perl -Mthreads -E"
async{ warn threads->tid while sleep 1}; 
sleep 1; 
warn threads->tid; 
open STDERR, '>', 'aNewLog';  
warn threads->tid; 
sleep 3
" 2>oldLog

C> type oldLog
1 at -e line 1.
0 at -e line 1.

C> type aNewLog
0 at -e line 1.
1 at -e line 1.
1 at -e line 1.
1 at -e line 1.
Perl exited with active threads:
        1 running and unjoined
        0 finished and unjoined
        0 running and detached