my %hash_1 = ();
$hash_1{"michael"} = "michael";
$hash_1{"sasi"} = "other";
$hash_1{"wife"} = 0;

my $thr = threads->create(\&hello,"michael",\%hash_1)->join;
my $thr1 = threads->create(\&hello,"sasi",\%hash_1)->join;

print " ".$hash_1{"wife"}."\n"; # will print '0' although it was manipulated in sub routine 'hello'

sub hello{
    my ($who,$hash) = @_;
    print  "hello from thread: $who ".$hash->{$who}."\n";

    if(defined($hash->{"wife"}) && !$hash->{"wife"}){
        $hash->{"wife"} = "Deena";
        print " ".$hash->{"wife"}."\n";
    }
}