注意在perl5.8中，必须用threads->create来生成线程，和许多介绍旧版perl的书籍中的有区别
use Thread qw/async yield/;
my $done=0;
sub frob {
my $arg=shift;
my $tid=Thread->self->tid;
print "thread $tid:frob $arg\n";
yield;
unless ($done) {
yield;
$done++;
frob($arg+10);
}
}
my @t;
for my $i (1..3) {
push @t,threads->create(\&frob,$i);
}
for (@t) {$_->join}
print "done is $done\n";
