That said, I would personally advocate avoiding using threads unless you absolutely need to.

Most of my programs include server and other working threads.

Make each shared object keep track of how much it is used. In CLONE, increment the usage of each object that the thread will actually use. In DESTROY, decrement the usage of each object that the thread incremented. The key is that you have to decrement all of the things that you increment.

I tried it out, but it doesn't seem to be working better.

Threads are always a huge complication

Looks like you are right. The code below could work better. But... either Perl crashes or threads don't finish anymore. Very strange.

#!/usr/local/bin/perl

use threads;

use Time::HiRes qw/usleep/;

while( 1 ) {
    my $class = Class->new();
    threads->create( \&thread_sub, $class, 0 );
    usleep 200_000;
}

sub thread_sub {
    my( $class, $level ) = @_;
    print "THREAD ", threads->self->tid, ", $level running\n";
    threads->create( \&thread_sub, $class, $level + 1 ) unless $level;
    usleep 400_000;
    print "THREAD ", threads->self->tid, ", $level finished\n";
    threads->self->detach;
}

1;

package Class;

use threads::shared;

our %MY_CXT : shared = ();
our $CXT_ID : shared = 0;

sub new {
    my $class = shift;
    my $this = {};
    $this->{'shared'} = &share( {} ),
    $this->{'shared'}{'refcnt'} = 1;
    $this->{'shared'}{'tid'} = threads->self->tid;
    $this->{'shared'}{'clone'} = 1;
    lock( %MY_CXT );
    $this->{'id'} = ++ $CXT_ID;
    $MY_CXT{$this->{'id'}} = $this->{'shared'};
    bless $this, $class;
}

sub CLONE {
    lock( %MY_CXT );
    while( my( $k, $v ) = each %MY_CXT ) {
        # do not CLONE anymore when it's destroyed in the main thread
        $v->{'refcnt'} ++ if $v->{'clone'};
        print "CLONE: id $k, refcnt $v->{'refcnt'}\n";
    }
}

sub DESTROY {
    my $this = shift;
    my $shared = $this->{'shared'};
    if( $shared->{'tid'} == threads->self->tid ) {
        # disable futher CLONEs
        $shared->{'clone'} = 0;
        # refcnt must decrement twice ?
        $shared->{'refcnt'} --;
    }
    $shared->{'refcnt'} --;
    print "DESTROY: id $this->{'id'}, refcnt $shared->{'refcnt'}\n";
    if( $shared->{'refcnt'} <= 0 ) {
        lock( %MY_CXT );
        # should not happen
        warn "already destroyed!!!" unless $MY_CXT{$this->{'id'}};
        delete $MY_CXT{$this->{'id'}};
    }
}

1;
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