sub ContextString
{
   my $context = shift();
   
   if($context eq "foo")
   {
      return "String 1";
   }
   elsif($context eq "bar")
   {
      return "String 2";
   }
}

my $t_vars = {
               context_sub => \&ContextString,
               other => "mmm donuts",
             };

# config opts are whatever you need them to be
my $template = Template->new($config_opts);

$template->process("template_file.tt",$t_vars) ||
   die $template->error();