#!/usr/bin/perl
use warnings;
use strict;
use Tk;
use Tk::Balloon;

my $mw = tkinit;
my $msg = 'press to change';
my $b = $mw->Button( -text => 'Power calculation',
        -command => sub{$msg = $msg . ' again'},
        -padx => 10,
     )->pack( -side => 'top',
       -anchor => 'center',
       -pady => 3,
     );

my $bln = $mw->Balloon( -background => 'LightYellow' )
                ->attach( $b, -msg =>\$msg );
MainLoop;