Are you getting a HANDLE or a struct? You keep saying handle, but you keep using a struct.

What's the signature of the DLL function? Is it

void __stdcall OpenBlockFile(
    const char* szFileName,   /* Input  */
    HANDLE*     pHandle       /* Output */
)
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If so, you need something like

my $dll = Win32::API->new(
      'db.dll', 'OpenBlockFile', 'PP', 'V' )
   or die $^E;

my $packed_handle = pack('L', 0);
$dll->Call($filename, $packed_handle);
$handle = unpack('L', $packed_handle);
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