perlguts, perlxs, and perlxstut manpages
Dean Roehrich's XS CookBookA and CookBookB: http://search.cpan.org/search?dist=CookBookA and http://search.cpan.org/search?dist=CookBookB
A series of articles at PerlMonth.com by Steven McDougall:
http://world.std.com/~swmcd/steven/perl/pm/xs/intro/index.html
http://world.std.com/~swmcd/steven/perl/pm/xs/tools/index.html
http://world.std.com/~swmcd/steven/perl/pm/xs/modules/modules.html
Chapters 18-20 in Advanced Perl Programming, by Sriram Srinivasan (O'Reilly)
Extending and Embedding Perl, by Tim Jenness and Simon Cozens (Manning Publications Company)
The perl-xs mailing list on http://www.perl.org/ (email perl-xs-subscribe@perl.org)
SWIG: http://www.swig.org/
Chapter 9 ("Tuning Apache and mod_perl") in mod_perl Developer's Cookbook, by Geoffrey Young, Paul Lindner, and Randy Kobes (Sams Publishing).
Mastering Regular Expressions: Powerful Techniques for Perl and Other Tools, Second Edition, by Jeffrey E. F. Friedl (O'Reilly)
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