You're mixing up database handle (usually $dbh) and statement handle (usally $sth) in this line:

$dbh = $dbh->prepare(...)
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That will place a statement handle (return value from $dbh->prepare) in a variable that you think contains a database handle. Another $dbh->prepare call goes BOOM.

So don't do that, use separate variables for separate things. There are working examples in the DBI docs.