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";