Try this one. I don't have nmap so I used something else that would produce output.

#!/usr/bin/perl

# http://perlmonks.org/?node_id=1176994

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

my $tester;

my $mw = MainWindow->new;
my $b1 = $mw->Button(-text => 'Go', -command => \&go)->pack();
my $b2 = $mw->Button(-text => 'Exit', -command => sub{exit} )->pack();

my $text = $mw->Scrolled('Text',-scrollbars=>'e')->pack;

MainLoop;

sub go
  {
  $tester = Tk::IO->new(
    -linecommand => sub {$text->insert('end', shift()) },
    );
  $tester->exec('ls -l | head'); # command goes in here
  }
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