Looks like the problem is the -F option to h2xs.  Not only is it useless here (which is why I dropped it), it's in fact the source of the problem, because it declares "factorial.h" to be a C preprocessor flag, so there remains no .h file to scan... 

 
$ h2xs -n Book::Factorial -A -O -x -F factorial.h
Defaulting to backwards compatibility with perl 5.10.1
If you intend this module to be compatible with earlier perl versions,
+ please
specify a minimum perl version with the -b option.

Writing Book-Factorial/ppport.h
Scanning typemaps...
 Scanning /usr/local/perl/5.10.1/lib/5.10.1/ExtUtils/typemap
Writing Book-Factorial/lib/Book/Factorial.pm
Writing Book-Factorial/Factorial.xs
Writing Book-Factorial/Makefile.PL
Writing Book-Factorial/README
Writing Book-Factorial/t/Book-Factorial.t
Writing Book-Factorial/Changes
Writing Book-Factorial/MANIFEST

----

$ h2xs -n Book::Factorial -A -O -x factorial.h
Defaulting to backwards compatibility with perl 5.10.1
If you intend this module to be compatible with earlier perl versions,
+ please
specify a minimum perl version with the -b option.

Writing Book-Factorial/ppport.h
Scanning typemaps...
 Scanning /usr/local/perl/5.10.1/lib/5.10.1/ExtUtils/typemap
Scanning factorial.h for functions...        # <--- !!
Scanning factorial.h for typedefs...
Writing Book-Factorial/lib/Book/Factorial.pm
Writing Book-Factorial/Factorial.xs
Writing Book-Factorial/Makefile.PL
Writing Book-Factorial/README
Writing Book-Factorial/t/Book-Factorial.t
Writing Book-Factorial/Changes
Writing Book-Factorial/MANIFEST

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The latter variant produces the follwing correct Factorial.xs file: 

#include "EXTERN.h"
#include "perl.h"
#include "XSUB.h"

#include "ppport.h"

#include <factorial.h>


MODULE = Book::Factorial        PACKAGE = Book::Factorial        


double
factorial_iterative_c(x)
    int    x

double
factorial_recursive_c(x)
    int    x

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In reply to Re^2: own perl module based on c library by almut
in thread own perl module based on c library by gants

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