weird interaction between `fork` and threads in perl 5 24.0

tf.pl

#!/usr/bin/env perl
use strict;
use warnings;
use 5.020;
use threads;

sub run_child {
    my ($value) = @_;

    my $tid = threads->tid;
    if (my $pid=fork()) {
        say "$tid waiting for $pid";
        # the waitpid() seems to block all threads, not just the
        # current one

        # my $exit = waitpid($pid,0);
        # say "$tid - $pid exited with $exit";
    }
    else {
        say "$$ spawned from $tid, value $value";
        say "$$ thread: ".$_->tid for threads->list;
        sleep(2);
        exit $value;
    }
}

my @threads = map { threads->create(\&run_child,$_) } 1..10;

while (my @running = threads->list) {
    say "$$ joining " . $running[0]->tid;
    $running[0]->join;
}

# this is to wait for the children to exit, since we can't use the
# waitpid
sleep(5);