For 15.5 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/Application:ERP:kivitendo/15.5/Application:ERP:kivitendo.repo
zypper refresh
zypper install perl-Imager
For 15.4 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/Application:ERP:kivitendo/15.4/Application:ERP:kivitendo.repo
zypper refresh
zypper install perl-Imager
For 15.3 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/Application:ERP:kivitendo/15.3/Application:ERP:kivitendo.repo
zypper refresh
zypper install perl-Imager