Win32::GUI can do this work for you :) The slightly modified code from its documentation:

#!/usr/bin/perl

use warnings;
use strict;

use Win32::GUI;

my $desk = Win32::GUI::GetDesktopWindow();
my $dw = Win32::GUI::Width($desk);
my $dh = Win32::GUI::Height($desk);

print "${dw}x$dh\n";
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Getting the size of all monitors can be difficult, as the desktop can be stretched or duplicated, and you cannot determine this easily :)