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

use Tk;
use Devel::Peek;

my $val = 0.20;

my $mw = 'MainWindow'->new;
$mw->Button(
            -text         => "\n Characteristics \n",
            -command      => \&room,
           )->pack(-fill => "x");
$mw->Button(
            -text         => "\n Calculate \n",
            -background   => "red",
            -command      => \&design,
           )->pack(-fill => "x");

MainLoop();

sub room {
    my $roomTl = $mw->Toplevel;
    $roomTl->Entry(
                   -textvariable => \$val,
                  )->pack(-fill => 'x');
}

sub design {
    my $Uf = 13;
    my $Uw = $val * $Uf; # HERE
    print Dump($val), "\t Uf = $Uf \t val = ${\$val} \t Uw = $Uw \n";
}