If you are trying to ditch the SvOK(SV*) ternary, you could add extra validation on the Perl side before calling c_func

For example:

  # Assuming this is within the *.pm package, and you are
  # only allowing a single parameter
  # as (what looks to be) defined in your XS
  c_func( ( ( defined( $_[0] ) ) ? $_[0] : '' ) );
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Where you can implicitly force the SvOK check in Perl-land. Though, this would simply move the ternary into Perl rather than in the XS; maybe what you are looking for.

Hope that helps!