Please, add this warning to 2.2 manual and ideally all previous versions too:
"Positional references with the $ sign are searched for and replaced by Zabbix agent only for flexible user parameters. For simple user parameters, such reference processing is skipped and, therefore, any $ sign quoting is not necessary."
It can be found in manuals >= v3.0 and is an important bit of information. I know because it could have saved me hours of troubleshooting.