$ perl -MLWP -e 'print "$LWP::VERSION\n"'
5.805

##</code><code>##

use WWW::Mechanize;
my $mech = WWW::Mechanize->new();

my $url = "file:///.../test.html";   # the mentioned form
$mech->get($url);
$mech->current_form->dump();
$mech->set_fields( '__EVENTTARGET' => 'mynewvalue' );
$mech->current_form->dump();

##</code><code>##

$ ./674272.pl
POST .../Browse.aspx?MyToken=632797078501515540 [aspnetForm]
  __EVENTTARGET=                 (hidden readonly)
  __EVENTARGUMENT=               (hidden readonly)
  __VIEWSTATE=/wEPDwUJ           (hidden readonly)
POST .../Browse.aspx?MyToken=632797078501515540 [aspnetForm]
  __EVENTTARGET=mynewvalue       (hidden readonly)
  __EVENTARGUMENT=               (hidden readonly)
  __VIEWSTATE=/wEPDwUJ           (hidden readonly)