I know it's not the situation that you have, but I've run into a problem where you can't be in the middle of a fetch and try to alter the database. Like-a-so:

$select = $dbh->prepare(q(select foo, bar from baz));
$delete = $dbh->prepare(q(delete from baz where foo=? and bar=?));
$select->execute();
$select->bind_columns(\($foo, $bar));
while($ref = $select->fetchrow_arrayref()) {
   $delete->execute($foo, $bar) if $foo == 3; #<----this won't work
}
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If you want to do something like that, you need a separate database handle for your "change" handle. So, I would need to make a second connection to the database to prepare the delete statement in this example.