Here's code from KinoSearch::Util::Carp, which calls Carp::confess from C.

void Kino_confess (char* pat, ...) {
    va_list args;
    SV      *error_sv;

    dSP;

    error_sv = newSV(0);
    
    va_start(args, pat);
    sv_vsetpvf(error_sv, pat, &args);
    va_end(args);

    ENTER;
    SAVETMPS;
    PUSHMARK(SP);
    XPUSHs( sv_2mortal(error_sv) );
    PUTBACK;
    call_pv("Carp::confess", G_DISCARD);
    FREETMPS;
    LEAVE;
}
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It assumes that somewhere, someone has invoked use Carp;.

If you know that either ISO or GNU variadic macros are available, you can transparently insert the filename and the line number of the C file from where the exception was thrown, plus the function if either __func__ (C99) or __FUNCTION__ (GNU) is defined. Take a look at KinoSearch::Util::Carp in my svn repository if you want to see how that can work.

The bonus contextual info is just plain ol' nasty C wrangling though -- the XS-specific stuff is the same. Check out perlcall for more info on how to call back to Perl from C.