You can find most (all?) of the relevant RFCs in a list at Application Protocols - TELNET

You might want to wade through the RFCs that have interesting titles, though you'll have to apply some judgement here ... (e.g. TELNET RANDOMLY-LOSE Option   TELNET SUBLIMINAL-MESSAGE Option)

I scanned through some of these looking for what I think you might want - the ability to tell clients "send me whatever you've got in your input buffer if more than X time has elapsed since last character entry".   None of the RFCs seemed to mention such a thing.

But I might be confused as your two paragraphs above seem to contradict each other?   You find character mode 'inconvenient' but are looking for a way to "instruct telnet clients to change their settings?"

Another bit of code that might be instructive is Net::Telnet which demonstrates handling Telnet option negotiation.