use Fcntl ':flock' ;

open(ONE, ">>C:\\temp\\RTEOlock.log") ;
flock(ONE, LOCK_EX | LOCK_NB) ;
# capture/store flock result here

flock(ONE, LOCK_UN) ;
close ONE ;

##</code><code>##

use Fcntl ':flock' ;

open(OUT, ">>C:\\temp\\RTEOlock.log") ;
flock(OUT, LOCK_EX) ;
...