use threads;;

open O, '>', 'junk.dat';;      ## Open a file
print O $_ for 1 .. 100;       ## print some stuff to it
print tell O;;                 ## print the file pointer position
392

async{                         ## spawn a thread
    print tell O;              ## inherits a (cloned) copy of the file handle 
392                            ## complete with pre-existing state

    print O $_ for 1 .. 100;   ## print some more stuff from within the thread
    print tell O;              ## And it's copy of the file handle reflects the change
784
};;

print tell O;;                 ## but back in the main thread the original doesn't see it
392

print O $_ for 1 .. 100;;      ## even after local modifications
print tell O;;                 ## 
784

close O; exit;;

c:\test>wc -l junk.dat         ## All the output ended up in the file,  
   300 junk.dat                ## But neither threads file handle 
                               ## reflects the true pointer position.