Perl is not installed by default on the router, but it certainly can be added using the Entware libraries.
Most of the scripts posted here, including my ad-blocking script, are shell scripts.
Asus and Asuswrt-Merlin use a limited BusyBox, which comes with the Ash shell.
If you want to learn shell scripting, start maybe here, with more info what the ash shell supports on the BusyBox website and here.
It depends how much you know. There are tons of free tutorials & books out there.
Like @thelonelycoder said, the particular scripting on AsusWRT-merlin is related to busybox & it's included ash. You could probably install bash though.
My favorite book is probably Unix Power Tools. My favorite online tutorial, for quite a few Unix-related topics, is probably http://www.grymoire.com/Unix/Sh.html