"$FindBin::Bin/lib64/perl5/5.8.5/x86_64-linux-thread-multi" expand to "/home/larry/project/sysmonitor/lib64/perl5/5.8.5/x86_64-linux-thread-multi"

I wander why there are two different path for the module, one is ..../lib64/perl5/site_perl/5.8.5/x86_64-linux-thread-multi/, another is ..../lib64/perl5/5.8.5/x86_64-linux-thread-multi/. one with 'site_perl', one is not. what does 'site_perl' mean?