You have a serious misunderstanding of what is going on. Tk::ExecuteCommand is designed to do just that .... execute a command , which means sending a valid command to a shell to be run. What you want to do is just plainly run some perl code and have it's output in the Text widget.

For the question of killing a running command, make another button labeled 'Stop', and use Tk::ExecuteCommand's $exec->kill_command;

my $kill_button = $mw->Button( -text => 'Stop',
                          -command=> sub{ $exec->kill_command;   }
                          )->pack();
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As to your other problem of sending plain text messages to the Text Widget, instead of a command, you will need to figure out a way in your subroutine of determining if you have a valid command, or just an informational message. If it's an information message, you can write it directly into the ROText subwidget like the following:

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

my $mw = MainWindow->new;

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

$ec->configure( -command => 'dir' );

$ec->execute_command;
$ec->bell;
$ec->update;

# read perldoc Tk::ExecuteCommand for the Advertised Widget section
my $ROText = $ec->Subwidget('text');
print "$ROText\n";

$ROText->configure(-bg=>'white');

$ROText->insert('end', "\n\nHi, this is a message\n\n");   
$ROText->see('end');


#execute some perl code and output it to the text widget
&do_something;

MainLoop;

sub do_something{
  # the Tk::ExecuteCommand kill button won't stop this

for (1..5){

  $ROText->insert('end', "$_\n");   
  $ROText->see('end');
   }
}
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