Algol68 had this property. Consider

  STRUCT (INT x, y) z = (1, 2)

Here z is a const struct. If you wanted a variable, you'd have to say

  STRUCT (INT x, y) z;
  z := (1, 2)

or equivalently

  REF STRUCT (INT x, y) z = LOC STRUCT (INT x, y) := (1, 2)

Basically a variable has a REF mode.