#!/usr/bin/perl
use warnings;
use strict;

use Tk;
use Proc::Background;

my $mw     = MainWindow->new;
my $msg    = $mw->Label(-text => 'Press the button once ready.')->pack
+;

my $button = $mw->Button(-text    => 'Start',
                         -command => \&run,
                        )->pack;

MainLoop();

sub run {
    my $proc = Proc::Background->new('kate'); # an editor
    $mw->update while $proc->alive;
    $mw->Label(-text => 'Finished.')->pack;
    $button->destroy;
}
[download]

You could even do timers

our @procs;
$mw->repeat( 100, \&diddleProcs );
MainLoop;
sub run { ... push @procs, $proc; }
sub diddleProcs {
    my @goners;
    my @alive;
    for ( @procs ){
      if( $_->alive ){ push @alive, $_; }
      else { push @goners, $_; }
    }
    @procs = @alive;
    PackGoners(\@goners);
}
[download]