First, I have to second marto in that you should always use placeholders, third (since second is already used) you could aggregate all your columns into one call: 

my @column_names = ("MO_resp","MT_resp","AO_resp");
my $sql = "select ".join ",", map { "sum($_)" } @column_names;
$sql .= " from ? where Date = ? and Hour like ?";
my $hour_sth = $hour_db->prepare( $sql );

$hour_sth->execute($table_name, $db_date, "$hour:%") or die $DBI::errs
+tr;

[download]

See join and map.
If you are iterating over $db_date and $hour, you should keep the call to $hour_db->prepare outside those loops to reduce the overhead of binding placeholders in the SQL statement.

perl -le'print map{pack c,($-++?1:13)+ord}split//,ESEL'

In reply to Re^7: using system command in regex by shmem
in thread using system command in regex by ravi45722

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