Names identify entities within a Fortran program unit (such as
  variables, function results, common blocks, named constants,
  procedures, program units, namelist groups, and dummy arguments).
  In FORTRAN 77, names were called "symbolic names".

  A name can contain letters, digits, underscores (_), and the dollar
  sign ($) special character.  The first character must be a letter
  or a dollar sign.

  In Fortran 95/90, a name can contain up to 31 characters.  HP
  Fortran allows names up to 63 characters.

  The length of a module name (in MODULE and USE statements) may be
  restricted by your file system.

  Note:
    Be careful when defining names that contain dollar signs.

    Naming conventions reserve names containing dollar signs to those
    created by HP.

  In an executable program, the names of program units, external
  procedures, common blocks, and modules are global and must be
  unique in the entire program.