Regarding the "&IsThaiLCons" versus "IsThaiLCons()" -- I've tried them both, and both produce the same error.

In that case it is highly likely that the module isn't exporting a sub by that name. Either modify the module to export the sub or else call it with the full package name in your script.

And don't forget that the first S in SSCCE stands for "Short". The shorter your failing script is, the easier it will be to find the problem.