my $colnum = 21;
my $sql_placeholders = '(?' . ',?' x $colnum . ')';

my $sql_insert_handle = 
    $dbh->prepare_cached(
        'INSERT INTO mytable '
        . $sql_placeholders
        . ' VALUES '
        . $sql_placeholders
    );
<...>
# yada yada yada
# generate comma-separated column names and values
<...>
$sql_insert_handle->execute($columns,$values);