Rather than passing the configuration variables in $config to process, as chargrill noted, you can specify these when creating the $tt object:

my $config = {
      INCLUDE_PATH => '/tt', 
      INTERPOLATE  => 1, 
      POST_CHOMP   => 1, 
      PRE_PROCESS  => 'header',
      EVAL_PERL    => 1,
};
my $tt = Template->new($config);
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and then specify the variables you want processed in your template in process:

sub main_page {
  
  my $output = '';
  my $template = 'main.tt';
  my $vars = { title =>'AJAX Login'};
  $tt->process($template, $vars, \$output)
       || die $tt->error();
  return $output;
}
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