I'm not sure but I think the following behavior is kind of inconsistent. 
Setting the environment works different on functions and program.

Case 1 and 2 export a variable before calling the env program. In case 2 
copying the assignment of HELLO works as expected. Case 3 and 4 export a 
variable before calling a function which calls the env program. In case 4 
copying the the assignment of HELLO doesn't work because HELLO is assigned to 
$HELLO but assigning HELLO to $WORLD (case 3) works.

Any reason why zsh behaves this way? Bash doesn't have this strange behavior. 



## 1: export to command (different variable)
$ WORLD=world
$ HELLO=$WORLD env | grep '^HELLO'
    HELLO=world

## 2: export to command (same variable)
$ HELLO=world
$ HELLO=$HELLO env | grep '^HELLO'
    HELLO=world


## 3: HELLO exported to shell function, same behavior as 1
$ call() { env | grep '^HELLO' }
$ WORLD=world
$ HELLO=$WORLD call
    HELLO=world

## 4: HELLO not exported to shell function, different from 2 and 3
$ call() { env | grep '^HELLO' }
$ HELLO=world
$ HELLO=$HELLO call
    HELLO=      ## <-- empty