Hspice subckt naming convention
The syntax for HSPICE is not well-defined, at least not well-documented. The manual states that it takes a type subnam.

Turning to the original SPICE, at least we find something:


  A name field must begin with a letter (A through Z) and cannot contain any delimiters.


However, what is a delimiter is left to your imagination. You have just found that . is a delimiter.

An unqualified guess is that you can get close by looking at the documentation for a lot of other programming languages, most notably Fortran and C. Identifiers must start with a letter or underscore, then continue with a sequence of letters, numbers or underscores.