I don't know of any sites, but get yourself a copy of Extending and Embedding Perl and join perlxs mailing lists. The mailing list is slow sometimes, but you will get some good help, as well as asking here.

I should have mentioned reading the man pages, perlxs & perlxstut. Here is nice site see how perl guts look :-), http://gisle.aas.no/perl/illguts/.