The Proc::WaitStat perl module does not have an ebuild yet, having one would be useful!
Pierre, for this bug and the previous, you should research g-cpan.pl (I think it's in the portage man pages), which will generate these ebuilds for you. Perl module ebuilds which are not dependencies of other packages are in general discouraged.
There's a man page for portage, just man g-cpan.pl - If the version of g-cpan on your box gives your trouble, give me a hollar, there's a newer version waiting to go out with the next portage bump.
I wanted to use it for AMP (Asterisk Management Portal) but g-cpan will do the job quite well. Sorry for the trouble.