One problem is your use of chdir. That changes the process's work directory, so it affects all your threads. This is bad, and probably the reason your open is failing.

Some improvements:

my $q = Thread::Queue->new();

my @threads =
   map {
      async { # Just nicer
         # You don't want dequeue_nb here!!!
         while ( defined( my $item = $q->dequeue() ) ) {
            # Don't kill the entire thread
            # if there's a problem with one job.
            eval { worker( $item ); 1 } or warn( $@ );
         }
      };
   }
      1 .. $thread_limit;

# Do this *after* the threads are created.
# Otherwise, you could have a deadlock.
# (Ok, not really since we don't place a size
# limit on the queue so the queue can grow
# to any size, but it's still nicer and
# allows us to limit the size of the queue.)
$q->enqueue( substr $_, 0, 6 ) for @file;

# Make it so the threads exit when the queue becomes empty.
$q->end;

$_->join() for @threads;
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