doh! I suspected they were tied handles, but I figured PerlIO_* would handle that. Otherwise, you end up with the following:

/* Returns TRUE on success. Sets errno on error. */
int my_print(SV* sv) {
    GV* const   gv = PL_defoutgv;
    IO*         io;
    MAGIC*      mg;
    PerlIO*     fp;
    int         success;
    const char* buf;
    STRLEN      to_write;

    io = GvIO(gv);
    if (!io && GvEGV(gv)) {
        io = GvIO(GvEGV(gv));
    }
    if (!io) {
        return FALSE;
    }

    mg = SvTIED_mg((SV*)io, PERL_MAGIC_tiedscalar);
    if (mg) {
        dSP;

        ENTER;
        SAVETMPS;

        PUSHMARK(SP);
        XPUSHs(SvTIED_obj((SV*)io, mg));
        XPUSHs(sv);
        PUTBACK;

        call_method("PRINT", G_SCALAR);

        SPAGAIN;
        success = 0 != POPi;
        PUTBACK;

        FREETMPS;
        LEAVE;

        return success;
    }

    fp = IoOFP(io);
    if (!fp) {
        return FALSE;
    }

    buf = SvPV(sv, to_write);
    while (to_write > 0) {
        int written = PerlIO_write(fp, buf, to_write);
        if (written < 0) {
            return FALSE;
        }

        buf      += written;
        to_write -= written;
    }

    return TRUE;
}
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(Untested. Based in part on code from pp_print, the function that implements the print operator.)

That writes the the currently selected handle. It shouldn't need much work to work with any handle. Why isn't this already supplied by Perl? Or did I overlook it?

Update: Arg! It's not complete. There are encoding issues.

If s is assumed to be bytes,

• If PerlIO_isutf8(fp) is true, s needs to be converted to UTF-8 (i.e. perform an utf8::upgrade).

If s is assumed to be UTF-8,

• If PerlIO_isutf8(fp) is false and the bytes pragma isn't in effect, s needs to be converted to iso-8859-1 (i.e perform an utf8::downgrade).

Does calling CORE::print work from XS? That would be simpler! If it doesn't work, it would still be simpler to write a Perl wrapper for print and call that.

Update: Fixed compilation errors. Didn't address the problem mentioned above.