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# save STDOUT
    open(my $STDOUT_ORIG, ">&", \*STDOUT) or die;
...
    #     this is connected via pipe with CHILD_PROC
    open(STDOUT, ">&", \*TO_CHILD_PROC);

# in thread
my $wh = $pipe->writer;
$wh->autoflush(1);
print $wh  "whatever \n"  ;