For this type of scenario, I usually store individual column names and values in a pair of arrays and join them, something like this

if(is_valid($form{'field1'})){
    push(@fields, 'field1');
    push(@values, $form{'field1'});
}
if(is_valid($form{'field2'})){
    push(@fields, 'field2');
    push(@values, $form{'field2'});
}

# repeat, with appropriate validity tests, for
# each field/column

$query = $dbh->prepare("insert into table (" .
                       join(',',@fields) .
                       ") values (" .
                       join(',', @values) .
                       ")");
$query->execute;
[download]