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

use Tk;

my $mw = 'MainWindow'->new(-title => 'nmap');
my $b1 = $mw->Button(-text => 'Go', -command => \&go)->pack;
my $b2 = $mw->Button(-text => 'Exit', -command => \&Tk::exit)->pack;

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

sub go {
    open my $nmap, '-|', 'nmap -A localhost' or die $!;
    $text->fileevent($nmap, 'readable', sub {
        $text->insert(end => scalar <$nmap>);
    });
}
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BTW, no need for sub {&go} etc. Lexical filehandles and 3-argument form of open exist for pipes, too.

($q=q:Sq=~/;[c](.)(.)/;chr(-||-|5+lengthSq)`"S|oS2"`map{chr |+ord
}map{substrSq`S_+|`|}3E|-|`7**2-3:)=~y+S|`+$1,++print+eval$q,q,a,
[download]