This is a long way shy of perfect, but it might help.

#! perl -slw
use strict;
use Win32::API;
use Win32::GuiTest qw[:ALL];
use Win32::GUI;

Win32::API->Import( 
    'user32', 
    'BOOL InvalidateRect( HWND hWnd, VOID, BOOL bErase )' 
) or die $^E;

my $hdc = new Win32::GUI::DC("DISPLAY") or die $^E;
my $pen = new Win32::GUI::Pen( -style => 0, -width => 1, -color => [ 2
+55, 0, 0 ] );

$hdc->SelectObject( $pen );

$hdc->BkMode( 1 );

my $oldHwnd;
while( my( $x, $y ) = GetCursorPos() ) {
    last unless $x or $y;
    my $hwnd = WindowFromPoint( $x, $y );
    Win32::Sleep 10 and next if $hwnd == $oldHwnd;
    InvalidateRect( $oldHwnd, 0, 1 );
    SendMessage( $oldHwnd, 0x0128, 0, 0 ) if $oldHwnd;
    my( $l, $t, $r, $b ) = GetWindowRect( $hwnd );
    print "$hwnd: $hdc ($x, $y) : ( $l, $t, $r, $b )";
    $hdc->MoveTo( $l, $t );
    $hdc->LineTo( $r, $t );
    $hdc->LineTo( $r, $b );
    $hdc->LineTo( $l, $b );
    $hdc->LineTo( $l, $t );
    $oldHwnd = $hwnd;
}
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