#!/usr/bin/perl  
use strict;  
use Win32::API;  
use Encode qw( decode );  

sub bstr_to_str {  
   my $bstr = $_[0];  
   my $len = unpack('V', unpack('P4', pack('J', $bstr)));
   my $raw = unpack("P$len", pack('J', $bstr+4));
   return decode('UTF-16le', $raw);
}

Win32::API::Struct->typedef('BSTR','INT','x','WCHAR','y','CHAR','z1','CHAR','z2');   
my $iSize=8 ;  
my $iReturn=\0;  
my $iReturn_buf = pack('l', $iReturn);  
Win32::API->Import('My.dll','BSTR RandomDec(LONG ,LONG *)')or die $^E;  
my $rv_bstr=RandomDec($iSize,$iReturn);  
my $rv = bstr_to_str($rv_bstr); ## how to unpack and manage BSTR?    
print($rv);