Although the following is probably overkill - plus it won't run as-is as a CGI script - I've wanted to test out code like this for a while now so I took this oppertunity to write this example up using Mojolicious. And in the process, hopefully show some of the advantages of modern web technologies and frameworks over CGI.pm :-) This can be run as a standalone server in development mode via morbo script.pl, and as a simple server via perl test.pl daemon.

#!/usr/bin/env perl
use 5.028;
use Mojolicious::Lite -signatures;
use Mojo::JSON qw/encode_json/;
use Mojo::Util qw/sha1_sum/;

# NOTICE: This script is designed to work in a single-threaded,
# single-process server only! (morbo or Mojolicious::Command::daemon)

get '/' => sub ($c) { $c->render(template => 'index') } => 'index';

my %runningprocs;

post '/submit' => sub ($c) {
    # form variables
    my $foo = $c->param('foo');
    my $bar = $c->param('bar');
    # set up the event dispatcher
    my $ee = Mojo::EventEmitter->new;
    # hash collisions theoretically possible but very unlikely (could 
+check `exists $runningprocs{$id}`)
    my $id = sha1_sum( time."\0".rand."\0".(0+$ee) );
    $runningprocs{$id} = $ee;
    $c->render(json => { eventurl=>$c->url_for('status', id=>$id) });
    # set up and run the subprocess
    my $subproc = Mojo::IOLoop->subprocess;
    $subproc->on(spawn => sub ($sp) {
            $ee->emit(status => { progress=>"Subprocess spawned in PID
+ ".$sp->pid }) });
    $subproc->on(progress => sub ($sp, @data) {
            $ee->emit(status => { progress=>\@data }) });
    # give client a second to connect to event source
    Mojo::IOLoop->timer(1 => sub {
        $subproc->run(
            sub ($sp) { return long_running_subprocess($sp, $foo, $bar
+) },
            sub ($sp, $err, @results) {
                if ($err) { $ee->emit(status => { error=>"$err", done=
+>"Error: $err" }) }
                else { $ee->emit(status => { done=>\@results }) }
                # don't clobber the event listener immediately (in cas
+e client took longer to re/connect)
                Mojo::IOLoop->timer(10 => sub { delete $runningprocs{$
+id} });
            });
    });
} => 'formsubmit';

get '/status/:id' => sub ($c) {
    my $id = $c->stash('id');
    my $ee = $runningprocs{$id} or return $c->reply->not_found;
    $c->inactivity_timeout(300);
    $c->res->headers->content_type('text/event-stream');
    $c->write;
    my $timerid = Mojo::IOLoop->recurring(10 => sub {
        $c->write(":\n\n") }); # comment as keepalive
    my $cb = $ee->on(status => sub ($ev, $data) {
        my $json = encode_json($data) =~ s/\n//gr;
        $c->write("event: status\ndata: $json\n\n");
    });
    $c->on(finish => sub ($c) {
        $ee->unsubscribe(status => $cb);
        Mojo::IOLoop->remove($timerid);
    });
} => 'status';

sub long_running_subprocess {
    my ($subproc, $foo, $bar) = @_;
    # this code is now running in the subprocess!
    $subproc->progress("Beginning work on Foo='$foo'");
    sleep 5; # ...foo...
    $subproc->progress("Finished work on Foo");
    if ( length $bar ) {
        $subproc->progress("Beginning work on Bar='$bar'");
        sleep 5; # ...bar...
        $subproc->progress("Finished work on Bar");
    }
    return "All done!";
}

app->start;

__DATA__

@@ index.html.ep
% layout 'main', title => 'Hello, World!';
<div>
%= form_for formsubmit => ( method=>'post', id=>'myform' ) => begin
<div>
%= label_for foo => 'Foo'
%= text_field foo => ( placeholder=>"Foo", required=>'required' )
</div><div>
%= label_for bar => 'Bar'
%= text_field bar => ( placeholder=>"Bar" )
</div><div>
%= submit_button 'Process'
</div>
%= end
</div>
<pre id="myoutput" style="padding:3px 5px;border:1px solid black;">
Output will display here.
</pre>
<script>
"use strict";
function addmsg(txt) {
    $(document.createTextNode(txt)).appendTo($('#myoutput'));
}
function getevents(url) {
    addmsg("Listening on "+JSON.stringify(url)+"\n");
    var events = new EventSource(url);
    events.onerror = function(err) {
        // the event apparently doesn't contain any details
        var errmsg = "Error connecting to EventSource";
        addmsg(errmsg);
        alert(errmsg);
        $("#myform :input").prop("disabled", false);
    };
    events.addEventListener('status', function (event) {
        var data = JSON.parse(event.data);
        if ( 'progress' in data ) {
            addmsg("Progress: "+JSON.stringify(data.progress)+"\n");
        }
        if ( 'error' in data ) {
            addmsg("Error: "+JSON.stringify(data.error)+"\n");
            alert(data.error);
        }
        if ( 'done' in data ) {
            addmsg("Done: "+JSON.stringify(data.done)+"\n");
            events.close();
            $("#myform :input").prop("disabled", false);
        }
    }, false);
}
$(function () {
    $('#myform').on('submit', function (e) {
        e.preventDefault();
        $("#myoutput").text("Submitting form\n");
        var thedata = $('#myform').serialize(); // before disabling!
        $("#myform :input").prop("disabled", true);
        $.ajax({ type: 'post', url: '<%= url_for 'formsubmit' %>',
            data: thedata })
            .done( function( data ) {
                getevents(data.eventurl);
            })
            .fail( function( jqXHR, textStatus, errorThrown ) {
                var errmsg = "Form submission error: "+textStatus
                    +" / "+jqXHR.status+" "+errorThrown;
                addmsg(errmsg);
                alert(errmsg);
                $("#myform :input").prop("disabled", false);
            })
    });
});
</script>

@@ layouts/main.html.ep
<!DOCTYPE html>
<html>
<head>
    <title><%= title %></title>
    <meta name="viewport" content="width=device-width, initial-scale=1
+.0">
    <link rel="stylesheet"
        href="https://cdnjs.cloudflare.com/ajax/libs/normalize/8.0.1/n
+ormalize.min.css"
        integrity="sha512-NhSC1YmyruXifcj/KFRWoC561YpHpc5Jtzgvbuzx5Voz
+KpWvQ+4nXhPdFgmx8xqexRcpAglTj9sIBWINXa8x5w=="
        crossorigin="anonymous" referrerpolicy="no-referrer" />
    <script src="https://code.jquery.com/jquery-3.6.0.min.js"
        integrity="sha256-/xUj+3OJU5yExlq6GSYGSHk7tPXikynS7ogEvDej/m4=
+"
        crossorigin="anonymous"></script>
</head>
<body>
%= content
</body>
</html>
[download]

If this needed to run in a threaded/multiprocess HTTP server, it would even be possible to replace the communication via EventEmitter objects with a system like Redis - it's pretty simple to spin up a server via Docker and connect to it using e.g. Mojo::Redis::PubSub.