I use a thread to be able to force an exit of the main thread

That is really, really, really bad design -- (*)like designing a train where the only way to get off is to smash a window and jump; and hoping that there'll be somewhere soft to land -- and it will come back to bite you!

* That's a pretty terrible analogy; maybe this is better: Like designing a satnav such that when you reach your destination, it kills your engine dead and then relies upon the anti-skid brakes and collision avoidance systems to bring you to a halt safely.

I seriously caution you against taking this design forward. Especially when it is so simple to avoid it:

use strict;
use threads;
use threads::shared;

my $inthread : shared;

sub doCleanUp {
    print "doing cleanup";
}

my $threadsleeptime : shared = 2;
my $mainsleeptime = 5;

my $thread = async {

    eval {
        $inthread = 1;

        sleep($threadsleeptime);
        print "Sleep in thread finished\n";

        die('abnormal exit');
    };
    if( $@ =~ /^abnormal exit/ ) {
        print "triggering cleanup in main\n";
        return -1;
    }
    return 0;
};

$thread->join() and doCleanUp() if ($thread);

print "End of main program\n";

__END__
C:\test>junk29
Sleep in thread finished
triggering cleanup in main
doing cleanupEnd of main program
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