use strict;
use warnings;
use Devel::Peek;
my $x = "";
Dump $x;
ddd($x);
print $x;
Dump $x;
use Inline Config => BUILD_NOISY => 1,
                    ClEAN_AFTER_BUILD =>0;
use Inline 'C' => 'Config' => ccflagsex => "-g";
use Inline 'C' => <<'CODE';
int ddd(SV* x){
    char* dx = "dddddd\n";
    printf("string dx is %p \n", dx);
// You have to use sv_use* family, since these functions free the previous PV in SV first for you, but don't use sv_usepvn otherwise perl will realloc pv. 
    sv_usepvn_flags(x, dx, 8, SV_HAS_TRAILING_NUL);
// Thanks ikegami, must set LEN to 0, otherwise, perl will try to free SV when leave scope.    
SvLEN_set(x, 0);  
    return 0;
}

CODE