Are you sure it's an error, not just a warning?   __EVENTTARGET presumably is a hidden field, which, according to HTML::Form, starts out readonly:

$bool = $input->readonly
$input->readonly( $bool )

This method is used to get/set the value of the readonly attribute. You are allowed to modify the value of readonly inputs, but setting the value will generate some noise when warnings are enabled. Hidden fields always start out readonly.

Ive tried setting the __EVENTTARGET and am still getting the warning. I have changed the link number to vary the results with no luck. Here is my code

use strict;
use WWW::Mechanize;
use Time::Local;
use POSIX 'strftime';
use HTML::Strip;

my($url) = 'http://www.usgbc.org/LEED/Project/RegisteredProjectList.as
+px?CMSPageID=243&CategoryID=19&';
my($pageCheck) = "";
my $mech = WWW::Mechanize->new(autocheck =>1);
my($dirname) = "C:\\Program Files\\Perl Express\\Scripts\\";
my($fname) = "output.txt";
my($x) = 0;
open(OUT , ">$dirname$fname") || die "Cant open file";

#initialization to strip HTML from document
my $hs = HTML::Strip->new();

$mech->agent_alias('Linux Mozilla');

$mech->get($url) or die "Page $url can't be reached";
print "Made it past the url test\n";

my($form) = $mech->forms();
#selects all options from the lstLeedRating Field
my $menu = $form->find_input("lstLeedRating");


#clicks the BtnSearch
$pageCheck = $mech->click_button(name => "btnSearch");

my $pageCheck2 = "";

$pageCheck2 = $mech -> submit_form(
    with_fields => {
        '__EVENTTARGET' => 'dgRegProjectList:_ctl29:_ctl7',
        },
        );

    if($pageCheck2->is_success){

             print $pageCheck2->content;
             #print $pageCheck->content;
             }
        else {
             print STDERR $pageCheck2->status_line, "\n";
             die "Page with those fields not found!";
             }

        my $page = $mech->content;

    my $clean_text = $hs->parse( $page );

        $hs->eof;

    $clean_text =~s/.*(multiple LEED Rating Systems)//s;
        my @lines = split /\n/, $clean_text;
    for (@lines){
            s/^\s+//;
            s/\s+$//;
            s/\s+/ /g;
    }
        $clean_text =~s/(2  3  4).*//s;
        my @lines2 = split /\n/, $clean_text;
    for (@lines2){
            s/^\s+//;
            s/\s+$//;
            s/\s+/ /g;
        }
    my $clean = join "\n", @lines2;

    print OUT $clean;

        close OUT;
[download]

I have a feeling it has somethign to do with there call to the __doPostBack

<script language="javascript" type="text/javascript">
<!--
    function __doPostBack(eventTarget, eventArgument) {
        var theform;
        if (window.navigator.appName.toLowerCase().indexOf("microsoft"
+) > -1) {
            theform = document.frmRegisteredProjectList;
        }
        else {
            theform = document.forms["frmRegisteredProjectList"];
        }
        theform.__EVENTTARGET.value = eventTarget.split("$").join(":")
+;
        theform.__EVENTARGUMENT.value = eventArgument;
        theform.submit();
    }
// -->
</script>
[download]

Let me know what you think, and thanks in advance!