But of course I now have the question...

I thought you might ask, but there's no point in expending energy until required :)

Instead of passing a handle to the object back to the parent so that it can control it, try giving the parent a shared variable that indicates what it wants the child to do to the object.

Try this. The thing to look for is $sig, named badly by analogy. Note also the sleep 1. Without it the parent thread will end the process before the child has chance to get a timeslice, with the result that you will create a zombie, which is actually quite hard to do on Win32:

#!perl -slw
use strict;
use threads;
use threads::shared;
use Thread::Queue;
use Win32::Process;
use Win32;

use Data::Dumper;

## A shared var to communicate progress between work thread and TK
my $Q = new Thread::Queue;

## A shared var to control the child process.
my $sig :shared = '';

sub work{
    # save STDOUT
    open(my $STDOUT_ORIG, ">&", \*STDOUT) or die;

    # connect TO_CHILD_PROC with CHILD_PROC
    pipe(CHILD_PROC, TO_CHILD_PROC);

    # change STDOUT to TO_CHILD_PROC
    # --> STDOUT of child process is set to TO_CHILD_PROC and
    #     this is connected via pipe with CHILD_PROC
    open(STDOUT, ">&", \*TO_CHILD_PROC);

    my $ProcessObj;
    Win32::Process::Create(
        $ProcessObj, "C:\\Perl32\\bin\\perl.exe", "perl child.pl",
        1, NORMAL_PRIORITY_CLASS, "."
    )|| die "$! : $^E";

    async {
        {
            $ProcessObj->Suspend if $sig eq 'Suspend';
            $ProcessObj->Resume  if $sig eq 'Resume';
            $ProcessObj->Kill(-1) and return
                if $sig eq 'Kill';
            Win32::Sleep 100;
            redo;
        }
    }->detach;

    # restore STDOUT
    open(STDOUT, ">&", $STDOUT_ORIG) or die;

    while( <CHILD_PROC> ) {
        chomp($_);
        $Q->enqueue( $_ );
    }

    close( CHILD_PROC );
}

threads->new( \&work )->detach;

use Tk;
use Tk::ProgressBar;

my $mw = MainWindow->new;
my $pb = $mw->ProgressBar()->pack();

my $repeat;
$repeat = $mw->repeat(
    100 => sub {
        while( $Q->pending ) {
            my $progress = $Q->dequeue;
            return unless $progress;
            $repeat->cancel if $progress == 100;
            $pb->value( $progress );
        }
    }
);

$mw->Button(
    '-text' => 'Cancel', '-command' => sub { $sig = 'Kill'; sleep 1; e
+xit 0; }
)->pack();

$mw->Button(
    '-text' => 'Suspend', '-command' => sub { $sig = 'Suspend'; }
)->pack();
$mw->Button(
    '-text' => 'Resume', '-command' => sub { $sig = 'Resume'; }
)->pack();

$mw->MainLoop;
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