Update, here's the code

#!/usr/bin/perl
use strict;
use warnings;
use HTML::TreeBuilder;
use WWW::Mechanize;
use URI::Escape;
my $agent = WWW::Mechanize->new();

$agent->get('http://www.usgbc.org/myUSGBC/Members/MembersDirectory.asp
+x?PageID=2011&CMSPageID=140');
$agent->form_name('frmMemberDirectory');
$agent->field('ddlCategory', '');
$agent->field('ddlState', '');
$agent->field('ddlCountry', 'US');
$agent->field('txtCompanyName', '');
$agent->click('btnSearch');

my $node = HTML::TreeBuilder->new();
$node->parse( $agent->content );

my $val_id = $node->find_by_attribute( id => '__VIEWSTATE');
my $validation = $val_id->attr('value'); 
my $state_id = $node->find_by_attribute( id => '__EVENTVALIDATION');
my $view_state = $state_id->attr('value');

$validation = uri_escape( $validation );
$view_state = uri_escape( $view_state );

my $id = '__EVENTTARGET=dgMembers%24ctl01%24ctl11&__EVENTARGUMENT=&__V
+IEWSTATE='.
         $view_state .
        '&txtCompanyName=&ddlCategory=&ddlState=&ddlCountry=US&__EVENT
+VALIDATION='. 
        $validation;

$agent->post('http://www.usgbc.org/myUSGBC/Members/MembersDirectory.as
+px?PageID=2011&CMSPageID=140',
            Content => $id);
print $agent->content();
[download]

problem solved

Change dgMembers%24ctl01%24ctl11 to dgMembers$ctl01$ctl10 and i'll get pagination 2, (for next paginations change to dgMembers$ctl01$ctl11)

my $id = '__EVENTTARGET=dgMembers$ctl01$ctl10&__EVENTARGUMENT=&__VIEWS
+TATE='.
         $view_state .
        '&txtCompanyName=&ddlCategory=&ddlState=&ddlCountry=US&__EVENT
+VALIDATION='. 
        $validation;
[download]

You was right. But you still don't need to use POST. This is work too:

#!/usr/bin/perl
use strict;
use warnings;
use WWW::Mechanize;
my $agent = WWW::Mechanize->new();

$agent->get('http://www.usgbc.org/myUSGBC/Members/MembersDirectory.asp
+x?PageID=2011&CMSPageID=140');
$agent->form_name('frmMemberDirectory');
$agent->field('ddlCountry', 'US');
$agent->click('btnSearch');

$agent->submit_form(
    
    form_name => "frmMemberDirectory",
    fields => {
        
        '__EVENTTARGET' => 'dgMembers$ctl01$ctl01',
        '__EVENTARGUMENT' => "",
    },
);

open my $test, ">", "test.htm" or die $!;
print $test $agent->content();
close $test;
[download]

encode isn't mentioned on that page at all.