You are setting $thread_endless_loop to the return value of the detach() method, which is not documented to return a value. Perhaps you meant:

my $thread_endless_loop = threads->create(\&endless_loop);

if (!($thread_endless_loop)) {
    my $message = "Main " . __LINE__ . ": Error";
    die("$message Cannot create a thread\n");
}
$thread_endless_loop->detach();
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Note also that $! will not have any meaningful value on failure to create a thread.

Dave.