You didn't say what operating system you're using. Is it Windows, Linux, Mac or something else?
If you're using Windows and you have ActiveState Perl, then XML::Simple is already installed as part of the ActiveState distribution.
If you're running Linux then the easiest thing to do is install the operating system package for XML::Simple. For example on Ubuntu you would use synaptic or apt-get to install the 'libxml-simple-perl' package.