SV = NULL(0x0) at 0x22cfb41ffb0
  REFCNT = 1
  FLAGS = (PADTMP)

##</code><code>##

SV = PV(0x22cfb55afc8) at 0x22cfb43c068
  REFCNT = 1
  FLAGS = (PADTMP,POK,pPOK)
  PV = 0x22cfd20c3d8 ""\0
  CUR = 0
  LEN = 10

##</code><code>##

use strict;
use warnings;
use Devel::Peek;

use Inline C =><<'EOC';

#include <string.h>
#include <stdlib.h>
 
const char *
string_reverse(const char *input)
{
  static char *output = NULL;
  int i, len;
 
  if(output != NULL) {
    free(output);
    printf("YES - output free()ed\n");
  }
 
  if(input == NULL)
    return NULL;
 
  len = strlen(input);
  output = malloc(len+1);
 
  for(i=0; input[i]; i++)
    output[len-i-1] = input[i];
  output[len] = '\0';
 
  return output;
}

const char *
nullstr() {
  return NULL;
}

EOC

print string_reverse("\nHello World");
Dump(string_reverse(undef));
Dump(nullstr());

__END__

Outputs:
dlroW olleH
Use of uninitialized value in subroutine entry at try.pl line 42.
YES - output free()ed
SV = PV(0x22b16cbafc8) at 0x22b16bdcba8
  REFCNT = 1
  FLAGS = (PADTMP,POK,pPOK)
  PV = 0x22b19158598 ""\0
  CUR = 0
  LEN = 10
SV = NULL(0x0) at 0x22b16bbfed0
  REFCNT = 1
  FLAGS = (PADTMP)