# Untested
foreach my $flavor qw( SPAM HAM ) {
  my $file = "/home/mail/mbox/${flavor}.mbox";
  $imap->select($flavor);
  my @msgs = $imap->search("ALL");
  foreach my $msg (@msgs) {
    open my $pipe, "| formail >> $file" or die "$!";
    $imap->message_to_file($pipe, $msg);
    close $pipe or die "$!";
    $imap->delete_message($msg);
  }
}