You should check out Perl Dancer. It is a micro framework that has session support built in, which allows you to preserve values across otherwise unconnected HTTP requests. I took a little liberty with your code and "ported" it over to work in a Dancer framework. Using templates is a better choice but CGI.pm still works -- you just want to be sure and not use the header() function, as Dancer will send that data for you. I will confess that i could not get this code to work with POST requests, only GET requests. That can be an exercise for the viewer. :) The issue has something to do with CGI.pm defaulting to "multipart/form-data" enctypes ... and since you really should use templates instead of CGI.pm the right thing to do is modify this code to use templates.

Also, don't forget to edit your config.yml file in your Dancer project to turn session handling on. Have fun!

package My::App;
use Dancer ':syntax';
use CGI qw(:all);

our $VERSION = '0.1';

# Initial variables
my @chars = ("A".."Z", "a".."z", "0".."9");
my $randomString;
$randomString .= $chars[rand @chars] for 1..8;
    
my @languages = ("English", "Francais");
my @colours = ("Red", "Orange", "Yellow", "Green", "Blue", "Indigo",
               "Violet", "Black");
    
get '/' => sub {
        
    session random_str => $randomString;
    my $random_str = session 'random_str';
    my $file = "$random_str.txt";
                     
    return 
        start_html() . 
        h2("Welcome to this basic form") .
        start_form( -action => 'select_color', -method => 'GET' ) .
        popup_menu({ -name => "language",
                     -values => \@languages,
                     -default => $languages[0] }) .
        p("The randomly generated string is: $random_str<br>The filena
+me is: $file") .
        submit({-value => "Continue"}) .
        end_form() .
        end_html()
    ;
};      
        
get '/select_color' => sub {
    my $random_str = session 'random_str';
    my $file = "$random_str.txt";
        
    return 
        start_html() .
        h2("Part Two") .
        start_form( -action => 'select_animal', -method => 'GET' ) .
        p("The randomly generated string is: $random_str<br>The filena
+me is: $file") .
        hidden({ -name => "language", -value => params->{language} }) 
+.
        "<p>Please choose your favourite colour: </p>" .
        radio_group({ -name => "colour",
                      -values => \@colours,
                      -default => $colours[0],
                      -linebreak => "true" }) .
        submit({-value => "Continue"}) .
        end_form() .
        end_html()
    ;
};      

get '/select_animal' => sub {
    my $random_str = session 'random_str';
    my $file = "$random_str.txt";

    return
        start_html() .
        h2("Almost Done!") .
        start_form() .
        p("The randomly generated string is: $random_str<br>The filena
+me is: $file") .
        hidden({-name => "language", -value => params->{language}}) .
        hidden({-name => "colour", -value => params->{colour}}) .
        p("Thanks for your help!") .
        end_form() .
        end_html()
    ;
};

true;
[download]

L-LL-L--L-LL-L--L-LL-L--
-R--R-RR-R--R-RR-R--R-RR
B--B--B--B--B--B--B--B--
H---H---H---H---H---H---
(the triplet paradiddle with high-hat)