I founded the key here:
If I increase the ref count of returned scalar, each time it will give me a different one:

void call_it() {
    ......
    SV* re = POPs;
    Perl_sv_dump(my_perl,re);
    SvREFCNT_inc(re);
    ......
}

int main(......) {
    ......
    // calling the first time
    printf("first time\n");
    call_it();
    
    // call again
    printf("again\n");
    call_it();
    
    // call again
    printf("and again\n");
    call_it();
    ......
}
[download]

$ gcc -g -o test -I /usr/lib64/perl5/CORE -L /usr/lib64/perl5/CORE -lp
+erl mini.c
$ ./test
going to return it: aaa
SV = PV(0xc43c78) at 0xc45d80
  REFCNT = 1
  FLAGS = (TEMP,POK,pPOK)
  PV = 0xc5bde0 "aaa"\0
  CUR = 3
  LEN = 8
again
going to return it: aaa
SV = PV(0xc43df8) at 0xc45f60
  REFCNT = 1
  FLAGS = (TEMP,POK,pPOK)
  PV = 0xc5c730 "aaa"\0
  CUR = 3
  LEN = 8
and again
going to return it: aaa
SV = PV(0xc43dc8) at 0xc612f8
  REFCNT = 1
  FLAGS = (TEMP,POK,pPOK)
  PV = 0xc6a290 "aaa"\0
  CUR = 3
  LEN = 8
[download]

Otherwise, it will give me the same one:

void call_it() {
    ......
    SV* re = POPs;
    Perl_sv_dump(my_perl,re);
    ......
}
[download]

$ gcc -g -o test -I /usr/lib64/perl5/CORE -L /usr/lib64/perl5/CORE -lp
+erl mini.c
$ ./test 
first time
going to return it: aaa
SV = PV(0x2001c78) at 0x2003d80
  REFCNT = 1
  FLAGS = (TEMP,POK,pPOK)
  PV = 0x2019de0 "aaa"\0
  CUR = 3
  LEN = 8
again
going to return it: aaa
SV = PV(0x2001c78) at 0x2003d80
  REFCNT = 1
  FLAGS = (TEMP,POK,pPOK)
  PV = 0x2019de0 "aaa"\0
  CUR = 3
  LEN = 8
and again
going to return it: aaa
SV = PV(0x2001c78) at 0x2003d80
  REFCNT = 1
  FLAGS = (TEMP,POK,pPOK)
  PV = 0x2019de0 "aaa"\0
  CUR = 3
  LEN = 8
[download]

Why??!!