use WWW::Mechanize;
use strict;

my $url = 'http://server/path/to/form.html';
my $mech = WWW::Mechanize->new();

$mech->get($url);
die "can't get page!" 
  unless $mech->success &&
         $mech->content =~ /look for something/;

$mech->submit_form(with_fields => {
     tree_fn  => 'learn',
     tree_ln  => 'perl',
     tree_gen => [ option => 'M' ]
  });
die "can't submit form!" 
  unless $mech->success && 
         $mech->content =~ /something useful/;