LongReadLen won't (as the OP seems to have discovered) help here:

The LongReadLen attribute only relates to fetching and reading long values; it is not involved in inserting or updating them.

I've never done anything with CLOBs either, but it strikes me that the bind and execute method calls both do the same thing--it might be worth either taking out the bind_param call or (more likely) removing the unnecessary argument from execute, so it's just so:

    my $sth = $dbh->prepare( $sql );
    $sth->bind_param(1, $xlist, { ora_type => ORA_CLOB });
    $sth->execute;
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If that works, my guess would be that the information from the first binding didn't make it to the database side before it was overridden, so the information that the variable was a CLOB went missing. If it doesn't, then I'm just as lost as AnonyMonk.