my $o = bless { a => 'A', b => 'B' }, "CLASS" ;

  show('main - pre ', $o) ;
  my $t = threads->new(\&proc, 'thread', $o) ; $t->join() ;
  show('main - post', $o) ;

  sub proc {
    my ($t, $x) = @_ ;
    $$x{z} = 'Z' ;
    show($t, $x) ;
  } ;

  sub show {
    my ($t, $x) = @_ ;
    printf "%20s: ", $t ;
    print "$x {", join(", ", map { "$_ => $$x{$_}" } sort keys %$x), "}\n" ;
  } ;