For timings set the "Profile" on each db handle:

$dbh->{Profile} = 6;
[download]

Change:

while ( @columns = $sth1->fetchrow_array ) {
    $inserted += $sth2->execute(@columns);
    ...
    }
}
[download]

to:

while ($row=shift(@$cache) || shift@{$cache=$sth_o->fetchall_arrayref(
+undef,$max_rows) || []}) {
    $inserted += $sth2->execute(@$row);
    ...
};
[download]

NB: Bunce says that optimal value for $max_row depends on the number and size of columns returned.