$widget->fileevent($fh, readable => sub {
    if (defined(my $read = scalar <$fh>)) {
        # Do something with $read here
    }
    else {
        $widget->fileevent($fh, readable => '');
    }
});

##</code><code>##

#!/usr/bin/env perl

use strict;
use warnings;
use autodie qw{:all};

use Tk;

{
    my $mw = MainWindow::->new(-title => 'Test `nmap` with Tk::fileevent');

    my $ctrl_F = $mw->Frame()->pack(-side => 'bottom');
    my $cmd_F  = $mw->Frame()->pack(-side => 'top');
    my $text_F = $mw->Frame()->pack(-side => 'top', -fill => 'both', -expand => 1);

    my $gui_data = {
        start_button_text => get_start_button_text(),
        stop_button_text  => get_stop_button_text(),
        button_pack_opts  => [-side => 'left', -padx => 10],
    };

    $gui_data->{out_T} = $text_F->Scrolled('Text',
        -scrollbars => 'osoe', -wrap => 'none', -bg => '#ffffff'
    )->pack(-fill => 'both', -expand => 1);
    sub get_button_width ();
    $gui_data->{start_B} = $cmd_F->Button(
        -textvariable => \$gui_data->{start_button_text},
        -state => 'normal',
        -width => get_button_width(),
        -command => [\&start_nmap, \$mw, $gui_data],
    )->pack(@{$gui_data->{button_pack_opts}});
    $gui_data->{stop_B}  = $cmd_F->Button(
        -textvariable => \$gui_data->{stop_button_text},
        -state => 'disabled',
        -width => get_button_width(),
        -command => [\&stop_nmap, \$mw, $gui_data],
    )->pack(@{$gui_data->{button_pack_opts}});

    $ctrl_F->Button(-text => 'Exit', -command => sub { exit })->pack;
}

MainLoop;

{
    my $nmap_running; BEGIN { $nmap_running = 0 }

    {
        my ($nmap_pipe, $nmap_pid);

        sub start_nmap {
            my ($mw, $gui_data) = @_;

            $nmap_pid = open $nmap_pipe, '-|', 'nmap -A localhost';
            $$mw->fileevent($nmap_pipe, readable => sub {
                if (defined(my $read = scalar <$nmap_pipe>)) { 
                    $gui_data->{out_T}->insert(end => $read);
                    $gui_data->{out_T}->yview('end');
                }
                else {
                    stop_nmap($mw, $gui_data);
                }
            });
            $nmap_running = 1;
            @$gui_data{qw{start_button_text stop_button_text}}
                = (get_start_button_text(), get_stop_button_text());
            $gui_data->{start_B}->configure(-state => 'disabled');
            $gui_data->{stop_B}->configure(-state => 'normal');
        }

        sub stop_nmap {
            my ($mw, $gui_data) = @_;

            $$mw->fileevent($nmap_pipe, readable => '');
            kill INT => $nmap_pid if kill 0 => $nmap_pid;
            $nmap_running = 0;
            @$gui_data{qw{start_button_text stop_button_text}}
                = (get_start_button_text(), get_stop_button_text());
            $gui_data->{start_B}->configure(-state => 'normal');
            $gui_data->{stop_B}->configure(-state => 'disabled');
        }
    }

    {
        my (@start_button_texts, @stop_button_texts);
        BEGIN {
            @start_button_texts = ('Start `nmap`', '`nmap` running ...');
            @stop_button_texts = ('`nmap` not running', 'Stop `nmap`');
        }

        sub get_start_button_text { $start_button_texts[$nmap_running] }
        sub get_stop_button_text { $stop_button_texts[$nmap_running] }

        sub get_button_width () {
            length +(sort { length $b <=> length $a }
                        @start_button_texts, @stop_button_texts)[0];
        }
    }
}