Perhaps your web server is using a different perl than you think it is. Perhaps it doesn't have read permissions to your library directory. Perhaps @INC is different under your web server than under your personal shell. Perhaps your web server doesn't have the module installed.

In any case, your issue appears to be that you're not getting XML::Simple from the context of your web server. Try some debugging or ask your server admin (or tech support) why you're having problems requiring XML::Simple since that's the problem.