I've had luck with something along the lines of:

XS(XS_widget)
{
    dXSARGS;

    FILE* in = PerlIO_findFILE(IoIFP(sv_2io(ST(0))));
    FILE* out = PerlIO_findFILE(IoIFP(sv_2io(ST(1))));

    /* copy in to out */
}
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Obviously you'd want to do stuff like error handling and arg counts and stuff, but that's basically how xsubpp handles XSUB definitions like the following in .xs files:

char*
read(fd,count)
   FILE* fd
   int count
  CODE:
   ...
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c.