Tk::IO lets you run another process and get callbacks for its output and completion.

#!/usr/bin/perl

# http://perlmonks.org/?node_id=1188639

use strict;
use warnings;
use Tk;
use Tk::IO;

my $status = 'startup';

my $mw = MainWindow->new;
$mw->geometry('+700+400');

$mw->Label( -textvariable => \$status,
  -font => 'Times 30', -fg => 'navy',
  -height => 3, -width => 20,
  )->pack;

my $button = $mw->Button(-text => "step one", -command => \&fun,
  -font => 'Times 30',
  )->pack;

MainLoop;

sub fun
  {
  $button->configure(-text => "step two" );
  Tk::IO->new(
    -linecommand => sub {$status = shift()=~tr/\n//dr},
    -childcommand => sub {
      $button->configure(-text => "step three" );
      },
    )->exec("echo start sleep; sleep 2; echo sleep ended");
  }
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