wrong # of arguments"][table set monitor offline indef indef HTTP::respond 200 content "offline"]

iCall is just a scheduler, it's pretty much a crontab job that will "survive" upgrades and reboots since it's coded into the configuration files. 

Use tmsh show sys icall to display the runtime info, like this: 

----------------------------------------------------
Sys::iCall::Periodic Handler: *****-scheduler
----------------------------------------------------
Number of times handler signalled 15.4K
Number of times handler ran 15.4K
Creation time 06/16/23 16:38:15
Current status active
Time since last status change 06/16/23 16:38:15

In the article you provided, it runs this command: exec perl /shared/scripts/vpn_ccu_monitor.pl

- This files needs to exist, and as the article says it should have the execution permissions (chmod +x) 
- You should try to run this script manually at least once, before attempting to schedule it. This way you'll see if it runs properly, or if it returns errors. 
- One thing I noticed, the script calls for the #!/usr/bin/perl -w interpreter. 
I know that the perl interpreter was moved to /bin/perl in more recent versions (run which perl to find it), as I've had to correct this syntax on multiple scripts I was using. 
- I'm not sure if the script returns anything into output. It's a good idea to add some "echo $variable" commands during the debugging, to confirm everything works properly.