You're getting your server stuff confused with your client stuff. Anything that happens dynamically on the page after it's loaded will be done by Javascript. The Perl script that creates the page will need to print that Javascript along with the HTML of the page. (Or it could be in a separate .js file that's included by the page.) So figure out what your Javascript needs to be by creating a static HTML test page with radio buttons that do what you want; then when that works, have your CGI print what you came up with.

I would highly recommend that you look into jQuery. It's a framework for doing dynamic page manipulation that sits on top of Javascript and eliminates a lot of the frustration of the language. This kind of thing is a one-liner in jQuery.

The solution to this problem would be to stop ignoring the advice you've been given when you've previously asked for JavaScript help.

• CGI button -onClick not working
• Javascript with CGI
• (OT) Javascript with CGI using different files
• (OT) javascript function not running when called from function with arguments

Yes, indeed.   Anything that happens “in response to a user-interface action on the user side” always is handled by JavaScript ... on the user side.   (The server does not become involved at all, unless the JavaScript initiates an AJAX request.)

The best-all-around way to handle appearance changes like this, in my humble opinion, is with CSS styles.   Your JavaScript code can query the object to determine which style it currently uses, remove that style and add the “next” one.   This allows you to change many characteristics of the object at one time and to do so consistently throughout.   There are plenty of examples on the Web on how to do this, and none of them involve Perl.

As other have noted, this hasn't anything to do with perl.

But since you're pretty close, try doing this:

#!/usr/bin/perl -w

use strict;
use warnings;
use CGI;

my $co    = new CGI;
my $color = 'red';

print $co->header;
print $co->start_html();
print $co->start_form;
print $co->radio_group(
        -name => 'color',
        -id   => 'color',
        -values => ['red','yellow','green'],
        -default => 'red',
        -onClick => "return select_color(this)"
    ).$co->p;
print $co->textarea(
    -id   => "textarea",
    -style=> "background-color:$color;
        color:blue",
);
print $co->end_form;
print $co->end_html;
print <<COLOR_SCRIPT;
<script language="JavaScript">
function select_color(rb_obj) {
    var color   = rb_obj.value;
    var ta_obj  = document.getElementById('textarea');
    ta_obj.style.background = color;
    return color;
}
</script>
COLOR_SCRIPT
[download]

Note that it passes "this" to the function select_color, so that the value will change with the color. The id of the text_area object is then looked up, and its style.background is assigned to the current color.