I'll somewhat disagree with Abigail-II and Elian.
Yes, if you want to interface Perl5 to a C library, then doing so via Inline::C is more likely to make it easy to port to Perl6 than using XS would (the XS porting might be available sooner, but you have much more risk of doing something that won't be handled if you use XS, IMHO).
Using Inline::C or XS to manipulate Perl data structures is something I simply don't recommend and I find that avoiding it when interfacing a library usually makes for a better design anyway.
C code that manipulates Perl data structures has always been the first thing to break when Perl is upgraded. So asking how to do that with Perl6 in mind calls the answer "Don't!" to mind (which is my answer anyway, just not as emphatically).
So when someone asks how to use XS with Perl 6 in mind, my advice is "Don't manipulate any Perl data structures in C and use Inline::C instead of XS." But, that is my advice anyway (again). And I'll bet that modules that follow that advice will eventually be able to work with Perl 6 without requiring any "by-hand" porting.