For WW2.18 in RHEL8 you don't need a custom version of perl from perlbrew. Mojolicious can be run using the version of perl available from dnf.
For your first issue, check that WW is using the perlbrew version of perl that has all of the modules installed (or better yet, get rid of the perlbrew perl and just use the standard perl).
When running hypnotoad directly from the command line, you are probably running it as a different user than the systemd script. I'm guessing the error about missing modules in this case is because the user doesn't have read permission for the WeBWorK files. Try running 'sudo -u [your_webwork_user] hypnotoad -f /opt/webwork/webwork2/bin/webwork2'
For your first issue, check that WW is using the perlbrew version of perl that has all of the modules installed (or better yet, get rid of the perlbrew perl and just use the standard perl).
When running hypnotoad directly from the command line, you are probably running it as a different user than the systemd script. I'm guessing the error about missing modules in this case is because the user doesn't have read permission for the WeBWorK files. Try running 'sudo -u [your_webwork_user] hypnotoad -f /opt/webwork/webwork2/bin/webwork2'