And here's a version with ->grid using -sticky => 'e' to right justify the labels.
Note no -anchor needed.

#!/usr/bin/perl

# http://perlmonks.org/?node_id=1185225

use strict;       # module to must always declare variables before you
+ use them
use warnings;     # module to show where is the error 
use Tk;           # module for the Windows GUI

my $mainwindow = MainWindow->new();
$mainwindow->geometry("600x150");
$mainwindow->title("Window");

# Disable the window Resize
$mainwindow->resizable(0,0);

# Menu display option
my $main_menu = $mainwindow->Menu();
$mainwindow->configure(-menu => $main_menu);

#File
my $file_menu = $main_menu->cascade(-label=>"File", -underline => 0, -
+tearoff=>0);
   $file_menu->command(-label=>"New", -underline=>0,
    -command=>sub{exit}, -state => 'disabled');
   $file_menu->command(-label=>"Exit", -underline=>0, -command=>sub{ex
+it});
   
# About
$main_menu->command(-label=>"About",
  -command=>sub{$mainwindow->messageBox(-title=> "About",
  -message=>"Version 3.0.0", -type => "ok")});

# text variable
my $label_firstname;
my $entry_firstname;
my $label_lastname;
my $entry_lastname;
my $label_loginid;
my $entry_loginid;
my $button_add;

# -anchor =>  'e' | 'w' | 'n' | 's' | 'ne' | 'nw' | 'se' | 'sw' | 'cen
+ter'

#            top
################################
# nw        n         ne #
#                   #
# w        center          e  #
#                   #
# sw        s         se #
################################
#        bottom

my $f1 = $mainwindow->Frame->pack; # Frame that will hold the grid

$label_firstname = $f1->Label(-text => 'Firstname:',
  )->grid(-row => 0, -column => 0, -sticky => 'e');
$entry_firstname = $f1->Entry(-width => 35,-text => 'Firstname',
  )->grid(-row => 0, -column => 1);

$label_lastname = $f1->Label(-text => 'Lastname:',
  )->grid(-row => 1, -column => 0, -sticky => 'e');
$entry_lastname = $f1->Entry(-width => 35,-text => 'Lastname',
  )->grid(-row => 1, -column => 1);

$label_loginid = $f1->Label(-text => 'Login ID:',
  )->grid(-row => 2, -column => 0, -sticky => 'e');
$entry_loginid = $f1->Entry(-width => 35,-text => 'loginID',
  )->grid(-row => 2, -column => 1);

$button_add = $mainwindow->Button(-text => 'Add New User',
  -command=>sub{exit})->pack(-anchor => 'se');

MainLoop();
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