That would be closer to

sub SvPVX { unpack 'J', pack 'p', $_[0] }

if (defined $y) {
    if (length($y)) {
        printf("Defined loc=%X, val='%s'\n", SvPVX($y), $y);
    }
    else {
        printf("Empty string: loc=%X\n", SvPVX($y));
    }
}
else {
    printf("Undefined (null ptr)\n");
}
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So you completely removed the case the OP was asking about.

The C equivalent would be closer to the following (memory leak aside):

Scalar* stringify(Scalar* sv) {
   if (scalar_type(sv) == undefined) {
       fprintf(STDERR, "Use of unitialised value\n");
       return NewEmptyStringScalar();
   } else if (scalar_type(sv) == string) {
       return sv;
   } else if (scalar_type(sv) == ...) {
       ...
   } else ...
}

int eq(Scalar* sv1, Scalar* sv2) {
   return strcmp(
      stringify(sv1)->string_buffer,
      stringify(sv2)->string_buffer
          ) == 0;
}
[download]