I have created the following test case to show how to preserve null (\0) characters during Perl/C string conversion. You need to use newSVpvn to convert C char * to Perl string while preserving '\0' in the char *. I suspect the Win32::API module uses newSVpvf or similar that does not preserve the null characters. I would roll my own ShareMem in XS instead of creating a separate DLL and accessing it via Win32::API.

# ----- Makefile.PL -----
use ExtUtils::MakeMaker;
WriteMakefile(
    'NAME'            => 'StringTest',
    'VERSION_FROM'    => 'StringTest.pm',
    'PREREQ_PM'        => {},
    'LIBS'            => [],
    'DEFINE'        => '',
    'INC'            => '',
);


# ----- StringTest.pm -----
package StringTest;

require 5.00503;
use strict;
use warnings;

require Exporter;
require DynaLoader;

our @ISA = qw(Exporter DynaLoader);
our @EXPORT_OK = qw(set);
our $VERSION = '1.00';

bootstrap StringTest $VERSION;

1;


# ------ StringTest.xs ------

#include "EXTERN.h"
#include "perl.h"
#include "XSUB.h"

MODULE = StringTest    PACKAGE = StringTest

SV *set(SV *s, IV l)
    CODE:
    {
      char *str = malloc(l);
      int i;

      memcpy(str, SvPVX(s, l), l);     // OK

      // debug the copy
      printf("(l=%d) ", l);
      for (i=0;i<l;i++) { printf("%02x ", str[i]); }
      printf("\n");

      RETVAL = newSVpvn(str, l);  // newSVpvn preserves null

      free(str);
    }
    OUTPUT:
    RETVAL


# ----- stest.pl ------
use strict;
use StringTest qw/set/;

my $str = "A\0B\0C\0";                 # a string with nulls

print "OLD: $str\n";

my $new_str = set($str, length($str)); # create a new string

print "NEW: $new_str\n";


# ------ OUTPUT ------
OLD: A B C
(l=6) 41 00 42 00 43 00
NEW: A B C
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