Unfortunately, I cannot affect the way the END block(s) are written.

Then add an END block in your code, prior to useing the modules (any modules), whos end blocks you wish to disable, and call thread->exit() within that END block. Eg.

#! perl ...
# main.script
my $originalPid = $$;
END{
    return if $$ == $originalPid;
    require threads;
    threads->exit;
}
use Foo;
use Bar;
...
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• By putting your END block ahead of any modules you use, you ensure that your END block is called first.
• If the curent pid == the pid when the code first ran, you are the parent, so quit early and allow the parent to run any END end blocks set up by the modules it uses.
• Otherwise, you are a child, so call threads->exit before any other END blocks are called.