use Win32::API::Prototype;   # von die infamous Dave Roth

ApiLink( 'user32.dll',
         'DWORD MessageBox( DWORD hwnd, LPTSTR lpText, LPTSTR lpCaption, DWORD wType)'
       ) || die;

$lpText = NewString("The text in the message box.");
$lpCaption = NewString("The Caption");

MessageBox(0,$lpText,$lpCaption,0);