Weird. You can simplify the code by removing the global $roomTl (which shortens roomSub), and you can also use a plain scalar to the same effect. I also added Devel::Peek to include some internal details. 
#!/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";
}

[download]

Interestingly, to make the behaviour correct, just change the "# HERE" line to 

    my $Uw = "$val" * $Uf;

[download]
لսႽ† ᥲᥒ⚪⟊Ⴙᘓᖇ Ꮅᘓᖇ⎱ Ⴙᥲ𝇋ƙᘓᖇ

In reply to Re: perl/Tk Problem with referenced variable in Entry by choroba
in thread perl/Tk Problem with referenced variable in Entry by contact@solamen.fr

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