My mips and x86 machines seemed to ignore this empty ewarn, but the portage (2.0.47-r7) on my sparc system hung on it. I had to edit the perl-5.8.0-r10 ebuild and remove the empty ewarns in the little note regarding threads before it would actually start unpacking the source. As such, this is why I'm sending this to the sparc team, unless some other arch has run into the same issue..
I found this is a general issue. The problem is that the info is also send to syslog using the /usr/bin/logger utility. This utility will read from stdin if there is no input on the command line. An empty string apparently does not qualify. This is a bug in the logging facility of portage. It should be smart enough to trow away empty warnings.
this is a portage problem
*** Bug 16727 has been marked as a duplicate of this bug. ***
*** This bug has been marked as a duplicate of 16466 ***