Here is a D::FV that works.

sub qs_process {
    my $self = shift;
    my $dbh = $self->param('mydbh');
    
    my ($results, $err_page) = $self->check_rm('qs_display', {
        required => [qw/upfirstname uplastname upemail upusername uppa
+ssword confirm_password/],
        optional => [qw/upzip upcountry/],
        filters => ['trim'],
        constraints => {
            uppassword => {
                #Make sure passwords are the same
                name => 'user_passwords',
                constraint => \&check_password,
                params => [qw(uppassword confirm_password)],
            },
            upusername => {
                #Is username unique?
                name => 'user_username',
                constraint => sub { !$self->check_username(lc $_[0]); 
+}
            },
            upemail => 'email',
            upzip => 'zip_or_postcode',
        },
        msgs => {
            any_errors => 'err__',
            prefix => 'err_',
            constraints => {
                'user_passwords' => "Your passwords don't match",
                'user_username' => "Username not unique",
            },
        },
    } );
    return $err_page if $err_page;

#
#Stuff here to handle case of validation being succesfull.
[download]

Don't forget that your template must have a set of TMPL_VAR fields that match the error messages. This code is not functional as it has a couple of external validation functions you can't see.

Here is a fragment of the template that is used for this form, note the 'err_' series of TMPL_VAR's in the third column.

<form method="post" action="/app/app02.cgi">
    <table border="0" cellpadding="0" cellspacing="2" width="490">
            <tr>
                <td class="lbl" colspan="3"><font size="-1">Using this
+ form you can sign up for preview privileges on our system. You will 
+be able to see everything a full member would see, the only thing you
+ cannot do is actually make appointments.</font></td>
            </tr>
            <tr>
            <td class="lbl" width="160">First Name</td>
            <td class="value" width="230"><input type="text" name="upf
+irstname" size="20"></td>
            <td class="err"><tmpl_var err_upfirstname></td>
        </tr>
        <tr>
            <td class="lbl" width="160">Last&nbsp;Name</td>
            <td class="value" width="230"><input type="text" name="upl
+astname" size="20"></td>
            <td class="err"><tmpl_var err_uplastname></td>
        </tr>
[download]

If you want to see it at work it is on a development server at: http://posiedon.mine.nu/app/app02.cgi?rm=mode_001

jdtoronto PS I am on my out for about 8 hours, but msg me if you want some help and I can help later.