use Win32::API;

Win32::API::Struct->typedef( RECT => qw{
                                LONG Left; 
                                LONG Top;
                                LONG Right;
                                LONG Bottom; });

Win32::API->Import("User32", "int GetDesktopWindow ()");
Win32::API->Import("User32", "int GetWindowRect ( int hWnd, LPRECT lpRect)");

my $h = GetDesktopWindow();
my $r = Win32::API::Struct->new('RECT');
GetWindowRect ( $h, $r );

print "Screen Resolution : ", $r->{Right} - $r->{Left}, ' x ',
                              $r->{Bottom} - $r->{Top};