Term::ReadLine::Gnu uses another trick, it calls the Tk event handler DoOneEvent() directly - with due care, because it blocks if there's no event dispatched. If the above techniques (or just the use of POE) aren't feasible, you could use that trick.

See Tk::Event.

update:

The proffered solution... Clock that goes backwards enhanced with a CPU-intensive, long running task. While the task is as silly as the clock itself, it is suitable for the POC.

It's just a matter of adding a signal handler and calling the event dispatcher in a non-blocking way.

#!/usr/bin/perl

use strict;
use Time::HiRes qw(ualarm);
use Tk;
BEGIN {*Tk::Clock::time = sub { time * -1 } }
use Tk::Clock;
my $wm = MainWindow->new;
my $ck = $wm->Clock->pack(-side => 'top');
my $b  = $wm->Button(-text => 'Start Process', -command => \&mainproce
+ss)->pack;
$ck->config(dateFormat => "dd-mm-yyy");

MainLoop;

#
# simulate a long running, CPU-intensive task
#
sub mainprocess {

    local $SIG{ALRM} = sub {
        DoOneEvent(Tk::Event::DONT_WAIT);
        ualarm(1000);
    }
    ualarm(1000);

    # task body begin

    my $c;
    $b->configure(-text => 'running...');
    while (1) {
        $c++; 
        print $c,"\n" unless $c % 1000;
        last if $c == 10000000;
    }
    $b->configure(-text => 'Start Process');

    # task body end

    ualarm(0);
}
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Pretty few lines of code added... well, I could shoehorn the alarm handler into one line, and it would be 3 lines - not counting the 'use Time::HiRes' line ;-)

I guess it's not necessary to offer a code conversion job for something that simple.

_($_=" "x(1<<5)."?\n".q·/)Oo.  G°\        /
                              /\_¯/(q    /
----------------------------  \__(m.====·.(_("always off the crowd"))."·
");sub _{s./.($e="'Itrs `mnsgdq Gdbj O`qkdq")=~y/"-y/#-z/;$e.e && print}