The support for RPC in glibc is to go away [1, 2] and it seems that dev-perl/Ace implicitly depends on it.
The way this dependence manifested itself for me was a build failure on a Gentoo Prefix on Mac -- which does not have glibc installed and uses the Apple's libc library instead. (The problem was not finding rpcgen binary, details to be attached) Emerging net-libs/rpcsvc-proto fixed this particular build failure -- however a generic solution is discussed in detail in 
Created attachment 503544 [details]
Created attachment 503546 [details]
emerge --info '=dev-perl/Ace-1.290.0-r2::local'
Created attachment 503548 [details]
emerge -pqv '=dev-perl/Ace-1.290.0-r2::local'
Created attachment 503550 [details]
The ebuild file used
The ebuild had to be modified slightly for Prefix-specific reasons. The change is the generation of "DARWIN_DEF" file within src_prepare(). That file defines the compiler and linker commands to be used on a Darwin (i.e. MacOS) system, and will be included by the Makefile. See https://bugs.gentoo.org/637102
Acknowledgements: Andreas K. Huettel <email@example.com> on gentoo-user mailing list, for helping understand the problem and find the solution.
The bug has been closed via the following commit(s):
Author: Andreas K. Hüttel <firstname.lastname@example.org>
AuthorDate: 2018-03-30 19:12:32 +0000
Commit: Andreas K. Hüttel <email@example.com>
CommitDate: 2018-03-30 19:12:47 +0000
dev-perl/Ace: Various fixes for glibc-2.26 and Darwin
Package-Manager: Portage-2.3.24, Repoman-2.3.6
dev-perl/Ace/Ace-1.920.0-r3.ebuild | 113 +++++++++++++++++++++++++++++++++++++
1 file changed, 113 insertions(+)