You're getting the undef warning via the SvPV_nolen macro. If you change that to use sv_setpvn you won't get that anymore. This... 
if (!SvOK(sv)) 
    sv_setpvn(sv, "", 0);

[download]

... is the XS equivalent of this... 

$sv = "" unless defined $sv;

[download]
--
Marvin Humphrey
Rectangular Research ― http://www.rectangular.com

In reply to Re^5: XS/Inline::C concat *any* two SVs. by creamygoodness
in thread XS/Inline::C concat *any* two SVs. by BrowserUk

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