amavisd-new needs IO/String.pm which is in dev-perl/IO-String. But this is not listed in the dependencies. Please add to the ebuild. (or if you currently have the error message on starting, emerge dev-perl/IO-String) The dependencies list dev-perl/IO-stringy. I don't know if it is really needed as it might have been included instead of dev-perl/IO-String. Maybe check/remove?
*** This bug has been marked as a duplicate of 73553 ***