Performs common operations on file specifications in a portable way. To do this, it automatically loads the appropriate operating system-specific module, which is one of File::Spec::Mac, File::Spec::OS2, File::Spec::Unix, File::Spec::VMS, or File::Spec::Win32. The complete reference of available functions is given in File::Spec::Unix; the functions are inherited by the other modules and overridden as necessary. Subroutines should be called as class methods, rather than directly.