use Thread qw(async);  
use threads::shared; 
 
my $a :shared; 
$a = 1;  
$thr1 = async {  
  my $b; 
  { 
  lock ($a); 
  $b = $a; 
  } 
  while($b==1) 
  { 
   opendir (DIR, "C:\\temp"); 
   my @filelist = grep (!/^\./, grep(-r, readdir(DIR))); 
   print ("file content is @filelist "); 
   close DIR; 
   exit "ERROR: file found!!\n" if (@filelist>0);  
   { 
    lock ($a); 
    $b = $a; 
    print ("Value of a is $a\n"); 
   }  
  } 
}; 
 
 
sleep (2); 
{ 
  lock ($a); 
  $a = 2; 
  print "\nSam EXP\n"; 
} 
$thr1->join;

# Bad 
use Thread qw(async);

 # Good 
use threads qw(async);

use Time::HiRes qw(usleep); 
# Sleep 0.00025 seconds or whatever you want 
usleep(250);