An ultimately doomed one that existed in Perl for a while was the pseudohash. They were regular integer-indexed arrays that could be accessed as though they were hashes (aka associative arrays / dictionaries). They even made it into the main Perl books at the time as this awesome time saving device. Except they weren’t.

I did a quick web search just now and someone did a talk about why they weren’t a great idea and they tell it better than I could; Link: https://perl.plover.com/classes/pseudohashes/

The supplied video doesn’t have great sound quality, and it might be better to just click through the slides under Outline at the bottom there.