Also see Re: use Strict: a rigorous way to break my command line variables?.
If XML::Simple is installed, use xml::simple; on Windows will not raise an error related to xml::simple or XML::Simple:
>perl -Mxml::simple -e "XMLin()"
Undefined subroutine &main::XMLin called at -e line 1.
>perl -MXML::Simple -e "XMLin()"
Could not find -e.xml in .\ at -e line 1
Because file systems on Windows are generally case insensitive, you can load modules under various names. As package names under Perl still are case sensitive, their ->import() method will not be called, which is confusing.
In my opinion, it would have been more helpful to directly tell the OP that there is no module xml::simple.