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

my $mbox = shift; # name of the mailbox file
my $mb = Mail::MboxParser->new( $mbox );
$mb->make_index;
my @index;
# Build index. I'm adding message position and subject
# into index, but you can add also other fields.
for ( 0 .. $mb->nmsgs - 1 ) {
    push @index, [ $_, $mb->get_pos($_), $mb->get_message($_)->header->{subject} ];
}
# save index into file
store \@index, "$mbox.idx";

##</code><code>##

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;
}