Thanks, that works! The syscall for my OS (arm-linux-gnueabihf-thread-multi-64int) is 224. The following works.

use threads;

sub gettid
{
    syscall 224
}

sub diag
{
    print "pid=$$ tid=@{[threads->tid]} gettid=@{[gettid]}\n";
}

async { diag }->detach for 1..5;
[download]

pid=16490 tid=1 gettid=16493
pid=16490 tid=2 gettid=16494
pid=16490 tid=3 gettid=16495
pid=16490 tid=4 gettid=16496