I would suggest using perl's ability to return multiple values. Your wrapper for getting the theme name could return both the error code and a regular perl string that way.

Update: Here is an example using Inline::C. You can examine the XS code it generates (a bit of ugly stuff, but it works).

use strict;
use Inline (C => Config =>
             CLEAN_AFTER_BUILD => 0);
 
use Inline C => <<'END_OF_C_CODE';
 
void twoout() {
   int first = 42;
   char *second = "abc";
   Inline_Stack_Vars;
   Inline_Stack_Reset;
   Inline_Stack_Push(sv_2mortal(newSViv(first)));
   Inline_Stack_Push(sv_2mortal(newSVpvn(second,3)));
   Inline_Stack_Done;
}
END_OF_C_CODE
 
my ($num, $str) = twoout();
print "num = $num, str = $str\n";
[download]