Experts will tell you to use Thread::Queue. I have not much experience with queues. However this is what you can achieve with shared variables:

use threads;
use threads::shared;

my ($global):shared;

my $handler=threads->create(\&handler)->detach;

do
    {
    
    print "Enter a value for shared global variable ( x = exit ): ";ch
+omp($global=<>);
    
    sleep(3);
    
    } while($global ne 'x');

sub handler
{

my $global_old=$global;

while(1)
    {

    if($global ne $global_old)
        {
        
        print "User changed the global variable to '$global'.\n";
        $global_old=$global;
        
        }

    sleep(1);

    }

}
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