/usr/lib/perl5/5.6.1
This is where the pure Perl modules that come as part of the standard Perl distribution are installed.
/usr/lib/perl5/5.6.1/i686-linux
This is where you'll find Perl modules that are part of the standard distribution and have an XS component (i.e. they need to talk to an external library).
/usr/lib/perl5/site_perl
You shouldn't find any modules in here.
/usr/lib/perl5/site_perl/5.6.1
This is like /usr/lib/perl5/5.6.1, but it's where modules that aren't part of the standard distribution go.
/usr/lib/perl5/site_perl/5.6.1/i686-linux
This is like /usr/lib/perl5/5.6.1/i686-linux but for modules that aren't part of the standard distribution.