Without some clues as to what is going on, it is impossible to diagnose the cause. And therefore difficult to suggest a solution. But I do understand your difficulty in providing those clues.

The single simplest help you could give us, is to post the code of a thread procedure that you know has silently died. If you have more than one, post them all.

One trick I've previously used to track down a similar problem is to wrap the thread procedure in a block eval. So for example, if you have code like this:

sub worker {
    ....
}

...

my @workers = map threads->new( \&worker, ... ), 1 .. $WORKERS;
[download]

Make the following simple modification:

sub worker {
    ....
}

sub proxy {
    my( $code, @args ) = @_;
    eval{ $code->( @args )   } or print $@;
}

...

my @workers = map threads->new( \&proxy, \&worker, ... ), 1 .. $WORKER
+S;
[download]

This may yield some clues. But posting the code of known to (sometimes) silently die thread subs would likely get better answers.