"...I have been wondering about the possibility of using PSGI/Plack without using any framework"

Me too. My 2 ¢:

# myapp.psgi

use Plack::Request;
use Plack::App::URLMap;
use HTML::HTML5::Builder qw[:standard];

my $slash = sub {
    my $env     = shift;
    my $status  = 200;
    my $request = Plack::Request->new($env);
    my $headers = [
        'Content-Type'   => 'text/plain',
        'Content-Length' => length $request->param('nose')
    ];
    my $body = [ $request->param('nose') ];
    [ $status, $headers, $body ];
};

my $tiny_monk = sub {
    my @things = ( "Sex", "Drugs", "Rock and Roll", "Charlie Haden", )
+;
    my $html = html(
        -lang => 'en',
        head( title('My Favorite Things'), Meta( -charset => 'utf-8' )
+, ),
        body(
            h1('Testomato'), p('My Favorite Things...'),
            ul( map li($_), @things ),
        ),
    );
    my $status  = 200;
    my $headers = [
        'Content-Type'   => 'text/html',
        'Content-Length' => length $html
    ];
    my $body = [ $html ];
    [ $status, $headers, $body ];
};

my $urlmap = Plack::App::URLMap->new;

$urlmap->mount( "/"          => $slash );
$urlmap->mount( "/tiny_monk" => $tiny_monk );

my $app = $urlmap->to_app;

__END__
[download]

Start the app with plackup myapp.psgi and try curl http://localhost:5000/?nose=cuke as well as curl http://localhost:5000/tiny_monk.

Please see also Plack::Request, Plack::App::URLMap and HTML::HTML5::Builder, map, Re^4: RFC: Proofread the POD for my HTML elements module and Re^3: RFC: Proofread the POD for my HTML elements module.

Best regards, Karl