Hm. Your code, with a couple of minor tweaks works fine for me:

#! perl -slw
use strict;
use threads;
use IO::Handle;

sub dummy {
    print "\n!!!!". $$;
    return 0;
}

open( my $handle, "| cat -v") || die "Unable to open connection to BT 
+Driver: $!\n";

$handle->autoflush(1);

#close $handle; If I uncomment this, threads can be joined. But I don'
+t want to terminate this child process.

my $thr2 = threads->create(\&dummy);

sleep 2;

print "\n$thr2";
for my $thr (threads->list(threads::joinable)) {
    print "\nIam here";
    print "\n!!!". $thr;
    print $thr->join();
}

__END__
C:\test>junk72

!!!!288

threads=SCALAR(0x37b820)

Iam here

!!!threads=SCALAR(0x367440)
0
[download]

Try it. Then try and work out why.