Sorry, I spoke too soon. They are indeed thread handles (at least 1000 of them are). And there is a way of avoiding the leak. Detaching the threads rather than joining them prevents it.

#!/usr/bin/perl -w

use strict;
use threads;

sub do_one_thread { warn "in kid\n"; return 1; }

sub do_thread {
   my $t = threads->new(\&do_one_thread);
   $t->detach;
}

for my $i (1..1000) {
   warn "i=$i:---\n";
   do_thread();
}
warn "sleeping for 60 seconds (check handle count in Task Manager)\n";
<STDIN>;
[download]

Of course thats no good if you need the return values from the threads, and so it is still a bug.