use strict;
use warnings;
use Mail::MboxParser;
use Storable;

my $mbox = shift;
my $num = shift;
# loading index
my $index = retrieve("$mbox.idx");

unless (defined $num) {
    # print index
    print join(" => ", @$_), "\n" for @$index;
}
else {
    # print message $num
    die "No such message" if $num >= @$index;
    my $mb = Mail::MboxParser->new( $mbox );
    $mb->set_pos($index->[$num][1]);
    print $mb->next_message_old;
}