Here's what ended up working:

open(OLDERR, ">&STDERR");
open(STDERR, ">>/tmp/tmp.spderr") or die "Can't dup stdout";
select(STDOUT); $| = 1;     # make unbuffered
print OLDERR "";  #this fixed an error about OLDERR not being used 

## do my stuff here.

close(STDERR);
open(STDERR, ">&OLDERR");
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As danger said, this is in perlfunc:open.

thanks,

melguin