As Autocommit is off you are required to finish every transaction with a commit or rollback. Unfinished transactions e.g. upon disconnect will not be committed.

    $dbh->do ("DELETE FROM managers WHERE id = '$self->{myId}'");
    $dbh->commit();     # <===== add this line
    $dbh->disconnect ();
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So just add the commit to your destroy code.

Update: looks like you found out in parallel :-)