This is because the entire expression is evaluated before being passed as arguments to print

No, the arguments are placed on the stack as they are evaluated.

1. $a = 2 is evaluated and the result ($a) is placed on the stack.
2. ' ' is evaluated and the result is placed on the stack.
3. $a = 3 is evaluated and the result ($a) is placed on the stack.
4. "\n" is evaluated and the result is placed on the stack.
5. print is evaluated.

@_ isn't even constructed here since print is an operator, not a sub.

The reason you get the observed behaviour is that $a itself (not a copy of it) is placed on the stack.