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

use Tk;

{   my $speed = 0;

    sub change_speed {
        my $diff  = shift;
        my $state = $speed <=> 0;
        if (! $state || $state == ($diff <=> 0)) {
            $speed += $diff if abs($speed + $diff) <= 100;
        } else {
            $speed = 0;
        }
    }

    sub create_label {
        my $parent = shift;
        return $parent->Label(-textvariable => \$speed,
                              -relief       => 'sunken',
                             )->pack(-ipadx => 5)
    }
}

my $mw = 'MainWindow'->new(-title => 'Motor');
my $mf = $mw->Frame->pack;

$mf->Label(-text => 'Tk Motor Controller')->pack;

my $l  = create_label($mf);

my $bf = $mf->Frame->pack;
$bf->Button(-text    => '<',
            -command => [ \&change_speed, -10 ],
           )->pack(-side => 'left');
$bf->Button(-text => '>',
            -command => [ \&change_speed, 10 ],
           )->pack(-side => 'right');

my $qb = $mf->Button(-text      => 'Quit',
                     -underline => 0,
                     -command   => sub { Tk::exit() },
                    )->pack( -pady => 10);
$mw->bind('<Alt-q>', sub { $qb->invoke });


MainLoop();
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Update: Used closure over the $speed variable.

($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,
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