This affects cyrus-imapd.
The package is built on a machine where Perl 5.22 is preinstalled, and consequently gets a dependency on perlapi-5.22.2. However, Debian Stretch ships Perl 5.24 - so the dependency is unfulfillable.
I thought this could be resolved by removing the line Preinstall: perl-modules-5.22 from the Debian:9.0 project configuration, but that doesn't seem to have done the trick.
I wonder why the OBS build machines have that old version of Perl in the first place - are they still stuck with an old prerelease of Stretch?