[lucio]

That syntax was irregular but very useful. It was not surprising or confusing.

A$ = "ABCD"

MID(A$,2,2)="12"

PRINT $A

>A123

[bbernoulli]

Assuming a typo here, but I would be surprised if the output contained "3"

[quietbritishjim]

This is still going strong today in list slice notation in Python (and other languages, no doubt):

    x = [1,2,5,6]
    x[2:2] = [3,4]
    print(x)

(Slice notation can be used on any collection, not just lists, but you can't use it in assignment like that for strings because they're immutable, so you'd have to convert to a list of characters and back.)