CGI::Ajax may be helpful. Here are a couple of quick examples to play around with. First is an example without javascript that prints to the page instead of a popup. (Update: More accurately said as "without having to write any javascript yourself", thanks almut) 
#! /usr/bin/perl

use strict;
use warnings;
use CGI;     
use CGI::Ajax;

my $cgi = new CGI;
my $pjx = new CGI::Ajax( 'exported_func' => \&too_many_nums );

print $pjx->build_html( $cgi, \&Show_HTML);

sub too_many_nums 
{
    my $output = "";
    my $input = shift;        
    if( length($input) > 10 )
    {
        $output = "Too many numbers!";
    }
    
    return( $output );
}

sub Show_HTML 
{
    my $html = <<EOHTML;
    <HTML>
    <BODY>
      Enter something: 
        <input type="text" name="val1" id="val1" onkeyup="exported_fun
+c( ['val1'], ['resultdiv'] );">
      <br>
      <div id="resultdiv"></div>
    </BODY>
    </HTML>  
EOHTML
  
    return $html;
}

[download]
And here is an example that uses a javascript popup. 
#! /usr/bin/perl

use strict;
use warnings;
use CGI;     
use CGI::Ajax;

my $cgi = new CGI;
my $pjx = new CGI::Ajax( 'exported_func' => \&too_many_nums );

print $pjx->build_html( $cgi, \&Show_HTML);

sub too_many_nums 
{
    my $output = "";
    my $input = shift; 
           
    if( length($input) > 10 )
    {
        $output = "Too many numbers!";
    }
    
    return( $output );
}

sub Show_HTML 
{
    my $html = <<EOHTML;
    <HTML>
    <BODY>
    <head>
        <script type="text/javascript">
            function disp_alert()
            {
                var input = arguments[0]
                if( input != "" )
                {
                    alert(input);
                }
            }
        </script>
    </head>

      Enter something: 
        <input type="text" name="val1" id="val1" onkeyup="exported_fun
+c( ['val1'], [disp_alert] );">
      <br>
      <div id="resultdiv"></div>
    </BODY>
    </HTML>  
EOHTML
  
    return $html;
}

[download]
hth

In reply to Re: Create pop up error message in Perl by hominid
in thread Create pop up error message in Perl by Rocko19

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