I received a request today for running MCE with Tk and displaying a progress bar. Your post was very helpful. Thank you.

use strict;
use warnings;

use threads;
use threads::shared;

use Tk;
use Tk::Font;
use Tk::ProgressBar;

use MCE;

# Shared variables
my $percentage_completed : shared = 0.0;

# Globals
my $percentage_label = 0.0;
my $download_label   = "Completed...$percentage_completed%";

# Main Program
my $mce_thread = threads->create(\&run_mce);

run_gui();

# Subroutines
sub indicator {
    my ($current_line, $total_lines) = @_;
    $percentage_completed = sprintf '%.1f', $current_line * 100 / $tot
+al_lines;
}

sub run_mce {
    my $n=10;
    my $mce = MCE->new(
        max_workers => 1,
        init_relay  => 0,
        sequence    => [ 0, $n ],
        chunk_size  => 1,
        user_func   => sub {
            my ($mce, $i, $chunk_id) = @_;
            MCE::relay { indicator($i, $n) };
            sleep 1;
        }
    )->run;
}

sub run_gui {
    my $mw = MainWindow->new(-title => 'Downloading...');

    my $message = $mw->Message(
        -textvariable => \$download_label,
        -width => 130,
        -border => 2
    )->pack(-side => 'top');

    $mw->geometry('350x100');
    $mw->resizable(1,0);

    my $progress = $mw->ProgressBar(
        -width => 15,
        -from => 0,
        -to => 100,
        -blocks => 50, #more block more smooth
        -gap => 1, #use 0 to get solid bar else use 1
        -colors => [ 0, '#104E8B' ],
        -variable => \$percentage_label
    )->pack(-fill => 'x');

    my $button = $mw->Button(
        -text => 'Cancel (ESC)',
        -command => sub {
            ($percentage_completed < 100.0)
                ? MCE::Signal::stop_and_exit('TERM')
                : $mw->destroy;
        }
    )->pack(-side => 'right');

    $mw->bind('<Escape>' => sub { $button->invoke });
    $mw->repeat(100 => \&update_gui);

    MainLoop;
}

sub update_gui {
    $percentage_label = $percentage_completed;
    $download_label   = "Completed...$percentage_completed%";
}
[download]