use strict;
use warnings;
use Tk;

my $roomTl;
my %window = (xAf=>0.20);

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

MainLoop;

sub roomSub {
  if (! Exists ($roomTl)) {   
    $roomTl = $mw->Toplevel(
    );
    $roomTl->Entry(
      -textvariable => \$window{xAf},
    )->pack(-fill => 'x');
  }
  else {
    $roomTl->deiconify();
    $roomTl->raise();
  }  
}

sub designSub {
  my $Uf = 13;
  my $Uw = $window{xAf} * $Uf;
  print "Uf = $Uf \t xAf = $window{xAf} \t Uw = $Uw \n";
}