yes, technically the form is submitted 'onBlur', but is only 'processed' when the submit button is hit.

If everything's OK at that point you can just return a 204 HTTP header.

I agree, it would be a nightmare, hence my suggestion that JavaScript was the best bet. I also agree that it can't be relied on for input verification, but if you use it to verify input of a form and still verify through CGI, it would be the most efficient as far as the user experience was concerned. Here's an example of how it could work:

#!/usr/bin/perl -w
use strict;
use CGI;
use CGI::Carp 'fatalsToBrowser';

my $q =new CGI;

# criteria for fields
my %regexp = (username        => '^\w+$',
              table_rows      => '^\d{1}$',
              table_cols      => '^\d{1}$'
              );
 
# check an input
if ($q->param('check_field')) {
  my $input = $q->param('check_field');
  check_input_ok($input);
  # ok, so do nothing
  print "Status: 204 OK\n\n";
  exit(0);
}

# process form
elsif ($q->param()) {
  process_form();
}

# present
else {
  show_form();
}

sub check_input_ok {

  my $input_name  = $_[0];
  my $input_value = $q->param($input_name);

  # check input fits criteria - simple eg here

  if ($input_value =~ /$regexp{$input_name}/) {
    # input's OK

    # represent form if input was table_rows or table_cols, and both v
+alid
    if (
         $q->param('check_field')
         &&
         $q->param('table_rows')
         &&
         $q->param('table_cols')
         &&
         ( 
           ($input_name eq 'table_rows') || ($input_name eq 'table_col
+s')
         )
       ) {
      show_form();
    }

  }
  else {
    # delete and re-present
    $q->delete($input_name);
    show_form("Input $input_name is invalid - please try again");
  }
}

sub show_form {
  # display form

  # grab error, if any
  my $err_message = $_[0];

  # reset check_field element
  $q->param('check_field','');  

  # here's the javascript
  my $js = <<'_END_';
    function check_input(fieldname) {
      document.testform.check_field.value = fieldname;
      document.testform.submit();
    }
_END_
  if ($err_message) {
    $err_message =~ s/'/\\'/;
    $js .= "alert('$err_message');\n";
  }


  print $q->header(),
        $q->start_html(),
        $q->script({-language=>'JavaScript'},$js),
        $q->start_form(-name=>'testform'),
        $q->hidden('check_field'),
        'Username: ',
        $q->textfield(-name=>'username','-onBlur',"check_input('userna
+me')"),
        $q->br,
        'Number of Rows(1-9): ',
        $q->textfield(-name   => 'table_rows',
                      -size   => 2,
                      -onBlur => "check_input('table_rows')"
                      ),
        'Number of Cols(1-9): ',
        $q->textfield(-name   => 'table_cols',
                      -size   => 2,
                      -onBlur => "check_input('table_cols')"
                      ),
        $q->p();

  # do we need to show a table?
  if ($q->param('table_rows') && $q->param('table_cols')) {
    # html to create table here...
    print $q->start_table({-border=>1}),
          $q->start_Tr,
          $q->th();

    # header row
    for (1..$q->param('table_cols')) {
      print $q->th("C$_");
    } 
    print $q->end_Tr;

    # content
    for my $row (1..$q->param('table_rows')) {
      print $q->start_Tr,
            $q->th("R$row");

      for (1..$q->param('table_cols')) {
        print $q->td("C$_ R$row");
      } 
      print $q->end_Tr;
    } 
    print $q->end_table();
  }

  # finish off form
  print $q->submit(-value=>'process',
                   -onClick=>"document.testform.check_field.value = ''
+;"),
        $q->end_form,
        $q->end_html;
  exit(0);
}

sub process_form {
  # check all fields again
  for (keys %regexp) {
    check_input_ok($_);
  }

  # everything submitted and checked, so continue
  print $q->header('text/plain'),
        'process form here';
  exit(0);
}

sub err {
  print $q->header('text/plain'),
        $_[0];
  exit(0);
}
[download]

This is only rough code, but you get the general idea...

cLive ;-)