Win32::API shows how to work with structures and perhaps this way one can eliminate some of those packs. So this is another angle (thankfully untested):

Edit: I have just edited this 3 mins after, because I was using the CryptProtectData

Win32::API::Struct->typedef( DATA_BLOB => qw{
  DWORD cbData;
  BYTE  *pbData;
}) or die;
Win32::API->Import('user32', <<EOP) or die;
BOOL CryptUnprotectData(
  DATA_BLOB                 *pDataIn,
  LPWSTR                    *ppszDataDescr,
  DATA_BLOB                 *pOptionalEntropy,
  PVOID                     pvReserved,
  CRYPTPROTECT_PROMPTSTRUCT *pPromptStruct,
  DWORD                     dwFlags,
  DATA_BLOB                 *pDataOut
)
EOP
# if all went well you have imported CryptUnprotectData()

my $datain = Win32::API::Struct->new('DATA_BLOB');
$datain->{'cbData'} = pack('LL', length($encryptedData)+1;
$datain->{'pbData'} = unpack('L!', pack('P', $encryptedData));
my $dataout = Win32::API::Struct->new('DATA_BLOB');
# call the imported func
my $result = CryptUnprotectData($datain, pack('L', 0), 0, pack('L', 0)
+, pack('L4', 16, 0, 0, unpack('L!', pack('P', 0))), 0, $dataout);
my $len = $dataout->{'cbData'};
my $dat = unpack('P'.$len, pack('L!', $dataout->{'pbData'}));
print "result ($len bytes) : '$dat'\n";
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