here's one script that outputs the popup menu _and_ handles the redirection. if you're not familiar with CGI.pm and it's methods for html generation, check out the CGI manpage.
note that you can either use a submit button to submit the form, or the onChange parameter to the popup_menu (requires javascript to work => bad idea), that submits the form automatically when you choose a class.

#!/usr/bin/perl
use warnings;
use strict;

use CGI;
my $q = CGI->new();

my %urls = ( 1 => "prog2.pl", 2 => "prog3.pl", 3 => "prog4.pl" );
my %labels = ( 1 => "Class 1", 2 => "Class 2", 3 => "Class 3" );

my $class = $q->param("class");

if ($class && $urls{$class}) {
    print $q->redirect($urls{$class});
} else {
    print $q->header, $q->start_html, $q->start_form;
    print $q->popup_menu(
        -name => "class",
        -values => [ keys %urls ],
        -labels => \%labels,
        # alternative to submit button below
        # -onChange => "submit()",
    );
    print $q->submit("go", "GO!");
    print $q->end_form, $q->end_html;
}
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snowcrash