To be able to determine the longest value, you need to get all of them. So, I'd use something like:

...
$sth->execute;
my $all = $sth->fetchall_arrayref;
my @max = (0) x 3;
foreach my $row (@$all) {
    for (my $i = 0; $i < @$row; $i++) {
        $max[$i] = length $$row[$i] if length $$row[$i] > $max[$i];
    }
}
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Now $max[0] contains the length of the largest ticket_id, $max[1] the length of the largest abstract, and $max[2] the length of the largest priority.