Please use code tags around your code, so it is more readable.

In addition to the technique of making $boom a global, you can also pass it to the subs, like this:

sub my_die {
   my $boom = MainWindow->new( -background => 'green' );
   my $label = $boom->Label();
   my $yes   = $boom->Button();
   my $no    = $boom->Button();
   
   $label->configure( -text => 'DO you really want to quit?' );
   $yes->configure( -text => 'Yes', -command => [\&des,$boom] );
   $no->configure( -text  => 'NO',  -command => [\&goback,$boom] );
   $label->pack();
   $yes->pack();
   $no->pack();
}

sub goback { 
   my $boom = shift;
   $boom->packForget(); 
   
}

sub des { 
   my $boom = shift;
   $boom->packForget(); 
}
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