I failed to mention that I cannot replace existing parameters with the new one I am appending. The above code does this. So I have written the below which appears to work:

use strict;
use URI::URL;

sub addEnvVar ($$$$)
{
    my $URL = shift;
    my $name = shift;
    my $value = shift;
    my $returnRef = shift;

    # Create the URL as an object and extract already existing parameters.

    my $url = new URI::URL ($URL);
    my $qstring = $url->equery ();

    # Split the parameters up.

    my @params = split (/\&/, $qstring);

    # Split the name=value pairs up and place them in a parameter list.

    my @paramList;
    foreach my $param (@params)
    {
	my ($pname, $pvalue) = split (/\=/, $param);
	push (@paramList, $pname);
	push (@paramList, $pvalue);
    }

    # Add the new parameters to the end of the list.

    push (@paramList, $name);
    push (@paramList, $value);

    # Place the parameter list in the URL.

    $url->query_form (@paramList);

    # Get the resulting URL after appending the new variable.

    my $newURL = $url->as_string ();
    $$returnRef = $newURL;

    return 1;
}

The only minor problem I had testing this occurred when I tried to have parameters such as:
   carr%20ots=cooked
These would appear in the resulting URL as:
   carr%2520ots=cooked
But I'm not sure if its even legal to have such things in
parameters.  I think I'll stick with this for now.
Thanks a lot for your help,

Leigh