This can't work:

  my $html_template = $self->param('html_template');
  ...
  $html_template->process('home', ... );
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So, where is the object supposed to come from? $html_template is, if set by the form, a string, so you're actually doing a class method call, most likely of a nonexistent package, and even if it exists, it most probably doesn't have a process method.

So there is where it goes wrong.

I think you still have to create a new html_template object from the parameter.