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");
}