Firstly covering the simpler of the two options, the ASGI/WSGI web server that goes between your Python code in the web framework and the users who have a web browser:

Gunicorn is the most popular WSGI server
Uvicorn is an ASGI server. It is both a standalone server as well as a plugin for Gunicorn
Hypercorn is an alternative that supports both WSGI and ASGI

Please put this into synopsys of 'plackup' $ SCRIPT_NAME='/' REQUEST_METHOD='GET' SERVER_NAME='www.test' SERVER_PORT='5000' plackup -s CGI t.pl

Also:

https://github.com/plack/Plack/issues/598
https://github.com/plack/Plack/issues/147
https://github.com/plack/Plack/pull/159/files
https://github.com/plack/Plack/pull/217
https://github.com/plack/Plack/pull/235

Moving from an old CGI application to Plack/PSGI in CGI mode and to running it in the Starman application server.

A modernisation of this using Plack is straightforwardly equivalent:

#!/usr/bin/env perl
use strict;
use warnings;
use Plack::Request;
use Encode::Simple;
use JSON::MaybeXS;

my $app = sub {
    my $req = Plack::Request->new($_[0]);
    my $input = eval { decode 'UTF-8', $req->parameters->{'input'} };
    [ 200, [ 'Content-Type', 'application/json; charset=UTF-8' ], [ encode_json { output => uc $input } ] ];
};

This is now a full PSGI app and as such gains all the same additional deployment options of the Mojolicious example. To deploy it as a CGI script you add this line at the bottom:

use Plack::Handler::CGI; Plack::Handler::CGI->new->run($app);

In all it takes about twice as long to load as the CGI.pm example, compared to the Mojolicious example taking about 7× as long. In exchange you get all the benefits of the PSGI ecosystem – without losing any of the strengths of deploying as a CGI script, unlike the Mojolicious example.