I think you mean "dialog" box. Obviously you are using CGI.pm so I would just create a form on index.html that contains at least 2 inputs, one for name and one for password, with a submit button that sends everything to your script for another round of validation.

The question I have is how users are getting to the code you show? Wouldn't that be from a form? How/where are you getting $valid_user in the first place? How about seeing more of what you have written?

Here I placed the "ind.cgi" coding and my home page "index.html" calling this cgi when user click the submit button.

#!C:\perl\bin\perl
use strict;
use CGI qw/:standard/;
use DBI;

my $q = new CGI;

my $dbh = DBI->connect("DBI:mysql:project", {PrintError =>0, RaiseErro
+r =>0}) ; 
my $us=$q->param('user');
my $pas=$q->param('pass');

my $sth = $dbh->prepare("SELECT * FROM user WHERE uname=? AND pass=PAS
+SWORD(?)");
$sth->execute($us,$pas);
my ($valid_user, $pp) = $sth->fetchrow_array();

if ($valid_user eq '')
{
    print redirect('http://localhost/index.html');        
}

print $q->header("text/html"),

$q->start_html(-title=>uc($q->param('REG')), -Background=>'EiffelTower
+.tif');
print "<br><br><H1 align='center'><b><font face='Euclid fraktur' color
+='cyan'>P</font><font face='Euclid fraktur' color='megantha'>E</font>
+<font face='Euclid fraktur' color='yellow'>R</font><font face='Euclid
+ fraktur' color='pink'>L</font> <font face='Euclid fraktur' color='Re
+d'>M</font><font face='Euclid fraktur' color='Green'>O</font><font fa
+ce='Euclid fraktur' color='blue'>D</font><font face='Euclid fraktur' 
+color='cyan'>U</font><font face='Euclid fraktur' color='megantha'>L</
+font><font face='Euclid fraktur' color='yellow'>E</font><font face='E
+uclid fraktur' color='orange'>S</font></H1>";
     
$dbh->disconnect;

print $q->end_html;
[download]

couldn't resist rewriting that huge print statement :)

my @colors = qw/ cyan megantha yellow pink Red Green blue cyan meganth
+a yellow orange /;
my $font = 'Euclid fraktur';
print "<br><br><H1 align='center'><b>";
foreach my $letter ( split //, 'PERL MODULES' ){
  do { print $letter; next; } if $letter = ' ';    # special case of s
+pace
  printf "<font face='%s' color='%s'>%s</font>", $font, $colors[0], $l
+etter;
  push @colors, shift @colors;                     # rotate the colors
}
print "</b></H1>";
[download]


As for your primary issue, I would suggest that index.cgi contain the login form (instead of a static index.html) so that you can dynamically add a message to it. Some pseudo code:

$message = '';
if( $login_form_submitted ){
  check user and pass params
  if ok
    redirect to main page
  else
    $message = 'login failed'
  end 
}
print login form here, including $message if it is set
[download]