In old school FORTRAN (I only recall WATFOR/F77, my uni's computers were quite ancient) subroutine (aka "subprogram") parameters are call-by-reference. If you passed a literal constant it would be treated as a variable in order to be aliased/passed by reference. Due to "constant pooling", modifications to a variable that aliased a constant could then propagate throughout the rest of the program where that constant[sic] was used.

"Passing constants to a subprogram" https://www.ibiblio.org/pub/languages/fortran/ch1-8.html