IMO dealing with NVs, IVs, UVs should be left to perl, its algorithms were tuned by years of everyday use and tons of bugreports. So instead of inserting your own "if numerix then stringify" better just do nothing and perl will DWIM.

I didn't start this for the fun of doing it. Believe me, nothing would please me more if that were the case, but it simply isn't. If I just use the API provided for concatenating 2 SVs, the result is no output--except a warning:

#! perl -slw
use strict;

use Inline C => << '__C__', NAME => 'test', CLEAN_AFTER_BUILD => 0;
#include <stdio.h>

SV* test( SV *a, SV *b ) {
    sv_catsv( a, b );
    return a;
}

__C__

for ( 1 .. 1e7 ) {
    print test( 'bill', 'fred' );

    my( $p, $q ) = ( 'fred' );
    print test( $q, $p );

    $q = 'bill';
    print test( $q, $p );

    $q = 1;
    print test( $q, $p );

    $p = 1;
    print test( $q, $p );
}
__END__
c:\test>test
Modification of a read-only value attempted at c:\test\test.pl line 25
+.

c:\test>
[download]

Try it. Nothing! No output at all from *any* of the tests. DWIM? Right.