If you use @filename to cause XPI to read from a script and it cannot find the script file in the current directory, XPI searches two other directories for the file. First, it checks to see if you have defined the environment variable MY_XCOMS. If MY_XCOMS is defined then XPI translates it and prepends the translation to the file name. Thus
setenv MY_XCOMS /users/my_name/my_xcoms
would cause XPI to search the directory /users/my_name/my_xcoms for the specified file.