That is not a simple thing to do.

If your function expects a variable length list of arguments of different types, the best approach would probably be to replace it by another function accepting a list of SVs, implemented using lower level functions.

Another option would be to implement the formating part in Perl, for instance:

sub FWriteF {
  my $file = shift;
  my $fmt = shift;
  my $text = sprintf($fmt, @_);
  _FRwriteF($file, $text);
     # calls C function as 
     # FWriteF($file, "%s", $text);
}
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If your function expects a variable list of arguments of the same type, sometimes it is acceptable to set a limit on the number or arguments so you can use something like this:

int
FWriteF(file, fmt, a0=0, a1=0, a2=0, a3=0,...., a10=0)
   FILE *file,
   const char *fmt
   int a0
   int a1
   ...
   int a10
CODE:
  switch (items - 2) {
    case 0:
      RETVAL = FWriteF(file, fmt);
      break;
    case 1:
      RETVAL = FWriteF(file, fmt, a1);
      break;
    case 0:
      RETVAL = FWriteF(file, fmt, a1, a2);
      break;
    ...
    case 10:
      RETVAL = FWriteF(file, fmt, a1, a2, a3, ..., a10);
      break;
  }
OUTPUT:
  RETVAL
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If I recall correctly, that's how SWIG does, for instance.