Body should be an array ref or filehandle: ... ##
##

#!/usr/bin/perl
use strict;
use Plack::Request;
use Plack::Builder;
use Plack::App::File;
use HTML::Template;

my $app = sub{
    my $env = shift;
    my $html = get_html();
    my $request = Plack::Request->new($env);

    if($request->param('file')){
        $html = thankyou();
    }

    return ['200', ['Content-Type'=>'text/html'],[$html]];
};

sub get_html{
    # open the html template
    my $template = HTML::Template->new(filename => 'scriptup.html');
    return $template->output();
}

sub thankyou{
    return <Thanks!
HTML
}