Inline is fine, but it is perhaps not as well integrated into ExtUtils and module building as the standard XS interface.

To answer your question directly, you should #include the perl headers before your C code, unless there is good reason not to. That gives you macros and functions to ease writing your functions . This example is adapted from perlxstut, which I recommend to you:

MODULE = Foo      PACKAGE = Foo

int
is_even(input)
        int     input
    CODE:
        RETVAL = (input % 2 == 0);
    OUTPUT:
        RETVAL
[download]

I'd parse the command line in perl before calling library functions with the arguments.

To link to an external lib, #include <foo.h> (its header) in the xs file, and put the linker directive '-lfoo' in Makefile.PL's LIB parameter.

After Compline,
Zaxo