Elvish's lambda syntax used to be [arg]{ ... }. But since [arg] is also the syntax for lists, it can get confusing - so I decided to move the argument list inside { ... }. And going through all the metacharacters Elvish already had, | was the one that was meaningless at the start of the lambda body, so I settled on {|arg| ... }.

I only realized the similarity with Smalltalk after that. I did learn a little Smalltalk many years ago but completely forgot about it, but it's possible that the knowledge was always somewhere in my subconscious brain.