the "OTRS user" would be the userid, under which the OTRS daemon (linux terminology) or service (windows terminology) is running. You have to find out which one that is. Might be the "local system", or a special user. The script wants to run under that same user. If it's "system", it might be easier to have the daemon changed to a special user instead (e.g. the otrs user you created)
There probably is a difference between things running from cmd.exe versus running from the linux subsystem. If the OTRS daemon is started from the WSL (Windows Subsystem for Linux), then you should do the same with the script.
Did the perl.exe come with OTRS, or did you install it yourself? Which flavour (Active, Strawberry, Cygwin, ...) and version is it? And most importantly: is it the same perl(.exe) that OTRS uses itself?