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

use Tk;

my $mw = tkinit();

my $txt_var;
my $spin = $mw->Spinbox( -from => 0, -to => 100, -textvariable => \$txt_var, -command => sub {print $txt_var . "\n"} )
->pack(-side => 'left');

my $cmd = $spin->cget('-command');
$spin->bind('<Return>',  $cmd);

MainLoop;