You have to be careful about the optionmenu widgets being in existence, before configuring their commands, when you want to set their state. This does what I think you want. It might also be good to have a Reset button so the button states can be restored, in case the user wants to redo the selections.

#!/usr/bin/perl -w
use strict;
use Tk;


my $mw = MainWindow->new();
    $mw-> geometry("400x100");
    $mw -> configure(-background => "cyan",
                    -foreground => "lightblue"
);


my ($var, $var1);

my $opt = $mw->Optionmenu(
        -background => "lightgreen",
        -options => [[jan=>1], [feb=>2], [mar=>3], [apr=>4]],
        -variable => \$var,
       )->pack;
$opt->addOptions([may=>5],[jun=>6],[jul=>7],[aug=>8]);

my $exit_but = $mw->Button(-text=>'Exit', 
    -command=>sub{
         print "Var= $var  Var1= $var1\n";
         $mw->destroy},
    -state => 'disabled',
    )->pack(-side=>"bottom");

my $opt1 = $mw->Optionmenu(
        -background => "lightgreen",
        -options => [qw(test1 test2 test3)],
        -variable => \$var1,
        -state => 'disabled',
       )->pack;

# must configure the $opt command AFTER $opt1 gets created
$opt->configure(-command => sub{
            print "got: ", shift, "\n";
            $opt->configure(-state => 'disabled');
            $opt1->configure(-state => 'normal');
            });

$opt1->configure( -command => sub{
         $opt1->configure(-state => 'disabled');      
         $exit_but->configure(-state => 'normal');      
            });


MainLoop;
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