The code I posted in the OP now does what I need it to do. Basically the XS equivalent of Perl's

$scalar1 .= $scalar2;
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I don't want to create a new scalar, except in the specific instance of a readonly input. The routine will be called many times and I need to accumulate the results in the scalar.

Duplicating the (already appended to), scalar in order to avoid using SvREFCOUNT_inc() is extremely wasteful and completely negates the benefits of perl's dynamically allocated string management. You have a scalar with n bytes, you call sv_catsv() to add m bytes to it. Having done that, you make a complete copy of m+n bytes in order to avoid the unreferenced scalar warning? This makes no sense to me, especially as the scalar of n bytes originates from Perl and has never gone out of scope.

My question still remains, why am I getting that warning?

The question is are there any other things that I need to test for and handle in order that I can handle any pair of SVs as input.

And if I comment out the sv_catsv() call ...

That is the (other) problem. If you call an SV that is undef as the first parameter to sv_catsv(), (which it usually will be on the first call for a particular SV), you get the warning. Hence the need for

    if( !SvPOK( a ) )      // Still nothing, must be undef?
        sv_setpv( a, "" ); // Make it the null string to stop (one pos
+sible)
        // Use of uninitialized value in subroutine entry from sv_cats
+v
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