Threads problem

I am getting the following issue with a first threads program I've written:

root@aixsys1:/usr/local/bin # ./threadtest.pl
Thread 1 with 0 and 10 to do something...
Can't call method "tid" without a package or object reference at
/usr/opt/perl5/lib/5.8.2/aix-thread-multi/threads.pm line 57.
A thread exited while 2 threads were running.

The script as is follows:

#!/usr/bin/perl

use threads;

sub mythread
{
my $arg1 = shift;
my $arg2 = shift;
my $myself = threads->self;
my $threadid = $myself->tid;

print "Thread $threadid with $arg1 and $arg2 to do something...\n";
sleep 30;
}

MYLOOP: foreach (0 ... 100)
{
if (threads->list == 10) {sleep 30; redo MYLOOP;}
$a=$_;
$b=$_ + 10;
$mythread = threads->create("mythread",($a,$b));
}

What am I doing wrong?

I want to keep 10 threads going at all times.

Thanks,
Philip