Moreover, your code does not use warnings and strict. You are happily using variables in go_page2 that come from the main programme body. Consider a cleaner version of the code:

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

use Tk;
use Tk::LabFrame;
use Tk::Dialog;
use Tk::NoteBook;


sub go_page2 {
    warn "<<@_>>";
    my ($nb, $entry_cell) = @_;
    my $name_topcell = $entry_cell->get;
    print $name_topcell, "\n";

    if ($name_topcell eq q()){
        print "yes  \n";
        my $mssg_empty = $nb->Dialog(-title          => 'Fill in Value
+s',
                                     -text           => 'Please fill i
+n all the values',
                                     -default_button => 'oke',
                                     -buttons        => [ 'oke'],
                                     -bitmap         => 'info',
                                    )->Show;
    } else {
        $nb->pageconfigure('page2', -state => 'normal');
        $nb->raise('page2');
    }
}


my $mw = MainWindow->new(-title => 'Main window');
$mw->geometry('1300x800');

my $nb = $mw->NoteBook->pack(-expand => 1,
                             -fill   => 'both');

####### Create page 1 ##########

my $entry_cell; # To be sent to go_page2.
my $p1 = $nb->add('page1',
                  -label => 'Input / Output');
$p1->Button(-text    => 'Next',
            -width   => 15,
            -height  => 2,
            -command => sub { go_page2($nb, $entry_cell) },
           )->place( -x => 1100, -y =>620);


my $btBrowse = $p1->Button(-text    => 'Browse',
                           -command => \&browse_file,
                          )->place( -x => 450, -y => 105);

my $btSave = $p1->Button(-text    => 'Browse',
                         -command => \&save_file,
                        )->place( -x => 1180, -y => 105);

my $entrybrowse = $p1->Entry(-width => 60)->place( -x => 20,  -y => 11
+0);
my $entrysave   = $p1->Entry(-width => 60)->place( -x => 750, -y => 11
+0);
$entry_cell     = $p1->Entry(-width => 20)->place( -x => 850, -y => 20
+0);

####### Create page 2 ##########
my $p2 = $nb->add('page2', -label => 'page2',
                           -state => 'disabled');


MainLoop();
[download]

Note that the user can still fill in the box (which probably should have a label of some sort), activate page 2, return back to page 1, clear the box and return to page 2 by clicking on its tab.