#!/usr/bin/perl

use warnings;
use strict;
use Win32::OLE;
use Win32::OLE::Const 'Microsoft Outlook';

my $msOutlook = Win32::OLE->new('Outlook.Application') or die $!;

my $nameSpace = $msOutlook->GetNamespace("MAPI");
my $folder    = $nameSpace->GetDefaultFolder(olFolderInbox);
my $items     = $folder->Items;

for my $itemIndex(1..$items->Count) {
    my $message = $items->Item($itemIndex);
    $message->{'UnRead'} = "False"; # $message->{'UnRead'} = "True"; ist das Gegenteil, also nicht 'Read' oder sonst etwas
} # for