Which critical blocks of code?

(This would go a lot quicker of you gave us some clue as to what you are trying to do! Ie. Post your code, or at the very least, some pseudo-code of what you're intending to do. As is, your question is impossible to answer.)

# create n numbers of threads
  #create ftp connection/login
  while (1)
  { wait msg Q();
    1. dequeue element
    2. download file from ftp server.
    3. delete file.
  }

# Master Thread
#create ftp connection/login
  while (1)
  { if 'files exist on ftp server'
    {
      enqueue files to download
    }
  }
[download]

To sum it, master threads gets list to download files and workers just downloads the file.

I am worried about multiple threads colliding, race condtions, in NET::FTP module.

So you don't really want a semaphore (people rarely need that), you just want a Thread::Queue for example, to distribute the jobs among your worker threads. Alternatively, if threads are unavailable to you, see Parallel::ForkManager, as your child/worker threads don't really need to communicate with the master thread.

Also, you might think about whether multiple threads will really improve the runtime of a process that is basically limited by the available bandwith and not by the available computation power.

I am worried about multiple threads colliding, race condtions, in NET::FTP module.

Just create a new instance of Net:FTP in each thread. There should(untested) be no conflicts.

This untested pseudo-code might get you started:

use threads
use Thread::Queue
use Net::FTP

my $Q = new Thread::Queue;

sub downloader {
    my $ftp = Net::FTP->new("some.host.name", Debug => 0)
      or die "Cannot connect to some.host.name: $@";
    $ftp->login("anonymous",'-anonymous@')
      or die "Cannot login ", $ftp->message;
    $ftp->cwd("/pub")
      or die "Cannot change working directory ", $ftp->message; 

    while( my $file = $Q->dequeue ) {
          $ftp->get( $file );
          $ftp->delete( $file );
    }
    $ftp->quit
}

my @threads = map threads->create( \&downloader ), 1 .. $N;

my $ftp = Net::FTP->new("some.host.name", Debug => 0)
  or die "Cannot connect to some.host.name: $@";
$ftp->login("anonymous",'-anonymous@')
  or die "Cannot login ", $ftp->message;
$ftp->cwd("/pub")
  or die "Cannot change working directory ", $ftp->message; 

$Q->enqueue( $ftp->ls ) or die $!;
$ftp->quit;

$Q->enqueue( (undef) x $N );
$_->join for @threads;
[download]

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