As AM points out above, in a very nice article I might add, you've linked to a tutorial whose Perl code is for Tkx. Here's a Tk version:

use Tk;
use Scalar::Util qw{looks_like_number};

my ($feet, $metres) = (q{}, q{});

my $mw = MainWindow->new(-title => q{Feet to Metres});
my $f = $mw->Frame()
    ->grid(-padx => 12, -pady => 3, -sticky => q{nsew});
my %pad = (-padx => 5, -pady => 5);
my $feet_E = $f->Entry(-textvariable => \$feet, -width => 7)
    ->grid(-row => 0, -column => 1, -sticky => q{we}, %pad);
$f->Label(-text => q{feet})
    ->grid(-row => 0, -column => 2, -sticky => q{w}, %pad);
$f->Label(-text => q{is equivalent to})
    ->grid(-row => 1, -column => 0, -sticky => q{e}, %pad);
$f->Label(-textvariable => \$metres)
    ->grid(-row => 1, -column => 1, -sticky => q{we}, %pad);
$f->Label(-text => q{metres})
    ->grid(-row => 1, -column => 2, -sticky => q{w}, %pad);
my $calc_B = $f->Button(-text => q{Calculate}, -command => sub {
    $metres = looks_like_number($feet)
        ? int(0.3048 * $feet * 10000.0 + 0.5) / 10000.0 : q{};
})->grid(-row => 2, -column => 2, -sticky => q{w}, %pad);

$feet_E->focus();
$mw->bind(q{<Return>} => sub {$calc_B->invoke()});

MainLoop;
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