gistfile1.PL

#!/usr/bin/perl

use strict;
use warnings;

use Mail::Box::Mbox;
use Encode;

use open ':std' => ':utf8';

my $inbox    = Mail::Box::Mbox->new(
  access => 'rw',
  folder => '/var/mail/willert',
  lock_type => 'NONE',
);

my @messages   = $inbox->messages;

my $i = 0;

for ( @messages ) {
  $_->head->set( 'X-Export-Status' => '1' )
    unless $_->head->get('X-Export-Status') or $i++;
  printf STDERR "Subject: %s [%s]\n", $_->subject,
    $_->head->get('X-Export-Status') || 'N/A';
}

$inbox->close;