I think that Data::Swap does exactly this. The Swap.xs file, once stripped of all checks, looks like:

#include "EXTERN.h"
#include "perl.h"
#include "XSUB.h"
MODULE = Data::Swap  PACKAGE = Data::Swap
void swap(foo, bar)
        SVref foo
        SVref bar
    PREINIT:
        void *any;
        U32 flags;
    CODE:
        any = foo->sv_any;
        flags = foo->sv_flags;
        foo->sv_any = bar->sv_any;
        foo->sv_flags = bar->sv_flags;
        bar->sv_any = any;
        bar->sv_flags = flags;
[download]

But for this you need to translate the .xs into a .c, compile the latter and place the resulting shared object in blib somewhere in the hierarchy. And you need also a Swap.pm module bootstrapping it.

I would like to avoid to ask people to install this module to run my code. Is it possible to hide these files in my user hierarchy?

I begin to believe that there is no native way to solve my original problem in Perl5.