use File::Find;
use Storable qw(store retrieve);

my $mbox = {}

foreach my $usr (@usr_dirs) {
    chomp $usr;
    my $usr_dir = $base_dir."/".$usr;

    undef $mbox;
    $mbox = retrieve($storable_file) if -e $storable_file;

    find(\&wanted,$usr_dir);
    store $mbox, $storable_file or die "Can't store !\n";
}