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.

Update:

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/.

In reply to Re^2: A problem converting C code to XS by sth
in thread A problem converting C code to XS by mbidwell

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