There is a very good module for doing this, called Tk::ExecuteCommand. Here is a basic example. You can do alot with this module, like pack it into different toplevel windows, instead of the mainwindow. A second example below does that. You can also change the appearance of the widget, by accessing it's "Advertised Subwidgets".

#!/usr/bin/perl
use warnings;
use strict;
use Tk;
use Tk::ExecuteCommand;

my $mw = MainWindow->new;

my $ec_dir = $mw->ExecuteCommand(
    -command    => 'dir; sleep 10; dir;',
    -entryWidth => 50,
    -height     => 10,
    -label      => '',
    -text       => 'Execute dir ',
)->pack;

my $ec_date = $mw->ExecuteCommand(
    -command    => 'date; sleep 10; date;',
    -entryWidth => 50,
    -height     => 10,
    -label      => '',
    -text       => 'Execute date ',
)->pack;


my $dir_but = $mw->Button(
               -text => 'Execute dir',
               -background => 'hotpink',
               -command => sub{ $ec_dir->execute_command })->pack;

my $date_but = $mw->Button(
               -text => 'Execute date',
               -background => 'lightgreen',
        -command => sub{ $ec_date->execute_command })->pack;

MainLoop;
[download]

For a separate toplevel:

#!/usr/bin/perl -w
use Tk;
use Tk::ExecuteCommand;
use Tk::widgets qw/LabEntry/;
use strict;

my $mw = MainWindow->new;

my $top = $mw->Toplevel;
$top->withdraw;

my $ec = $top->ExecuteCommand(
    -command    => '',
    -entryWidth => 50,
    -height     => 10,
    -label      => '',
    -text       => 'Execute',
                                  )->pack;
$ec->configure(-command => 'date; sleep 10; date');


my $button = $mw->Button(-text =>'Do_it',
                         -background =>'hotpink',
                         -command => sub{ $top->deiconify;
                                          $top->raise;
                                          $ec->execute_command;
                                          $top->withdraw},
                        )->pack;

MainLoop;
[download]