It is always helpful to show the exact command or GUI steps that was executed and failed, otherwise we are guessing.
The command below shows perl-json as a dependency.
pamac info clamtk
I just did an update recently and clamtk, in the community repository, shows version 6.14 and perl-json at version 4.10. Make sure to refresh your respositories and run an update, and then install. It your local repositories are not in sync with Manjaro’s that would cause 404’s too.