use strict;
use warnings;
use 5.010;
use threads;
use threads::shared;


my $flag = 0;
share($flag);
say "\$flag in main() is: $flag";


$SIG{'HUP'} = sub { 
    $flag = !$flag;
    say "signal caught, \$flag in main() changed to: $flag";
};


sub thr_func
{
    for (1 .. 5) {
        sleep 1;
        say "\$flag in thread is: $flag";
    }
}



my $thr = threads->create('thr_func')->detach();
sleep 3;  #allow time for thread to be created and start running

kill('HUP', $$);  #send signal to main()

sleep 10;   


--output:--
$flag in main() is: 0
$flag in thread is: 0
signal caught, $flag in main() changed to: 1
$flag in thread is: 0
$flag in thread is: 1
$flag in thread is: 1
$flag in thread is: 1