UPDATE:

I am having issues being able to accept() a connection from a process on a socket that I established (and written to) at a previous point in my script... in an nutshell, I am trying to match input given to me by a First process, if I match this input in the Second process I send it to the Third process to see if I have yet another match.

Sending the data from First, to Second, to Third works fine.

My problem comes in being able to establish a connection "downstream" (Third, to Second, to First) using the same set of sockets that I originally used to send data "upstream". In short, I cannot seem to get my $socket->accept to pick up the response from Third to Second, and then from Second to First.

Below is a chunk of code that I've managed to isolate as the hangup in my project:

while (<$first>) 
    {
        my $name = $_;
        chomp($name);
        
        #send acknowledgement message to First
        print "Received from [$f_host $f_port]: $name\n";
        #print $f_socket "$.: $name\n";

#Sending info upstream to Third process
#this part works fine        
        if($name eq $match)
        {
            print "Second found match: $name\n";
            print "Second sending \"$name\" to Third\n";
            
            print $t_socket "$match\n";
        }
        
#after I send I am expecting a response
#from Third process
#I never get into this loop
#it skips or accept() blocks indefinitely
        while(my $third = $t_socket->accept)
        {
            print "Second has accepted socket to Third...\n";
            
            my $t_host = gethostbyaddr($third->peeraddr, AF_INET);
                my $t_port = $third->peerport;
            
            while (<$third>)
            {
                my $name = $_;
                chomp($name);
                
                print "Received from [$f_host $f_port]: $name\n";
                
                if($name eq $match)
                {
                    print "Received \"$name\" from Third\n";    
                 
                     last;       
                }
                else
                {
                    last;
                }
            }    
        }        
                
    }
[download]

Here is the sending part of the Third process:

if($name eq $match)
        {    
            print "Third sending \"$name\" to Second\n";
            print $socket "$name\n";
            #close $second or die "Can't close ($!)\n";
            
            last;
        }
[download]

This code seems to execute okay.

I'd be happy to include all three scripts if anyone is interested to look. I'm doing this for a class project (see http://www.perlmonks.org/?node_id=472004 for a better description).

#####PREVIOUS QUESTION#####

I have a quick IO::Socket question for those of you out there familiar w/ Perl IPC.

What is the difference between opening a socket with "LocalPort" as opposed to "PeerPort" (see code example below):

my $socket = IO::Socket::INET->new('LocalPort' => $PORT,
                       'Proto' => 'tcp',
                       'Listen' => 2)
or die "Second: Can't create socket to First($!)\n";
   
#VERSUS

my $socket = IO::Socket::INET->
   new( 'PeerAddr' => "localhost",
    'PeerPort' => $PORT,
    'Proto' => 'tcp')
or die "Second: Can't create socket to Third($!)\n";
[download]

In this instance, can each of these $socket be used to write or read from? Is the former only for listening, and the latter for sending or can I use both sockets equally?

Can I only do a $socket->accept on the former example, but not the latter?

This leads me to my next question... what is the reason behind doing the following:

while(my $handle = $socket->accept)
{
  while(<$handle>)
  {
      #do something with $_
  }
}
[download]

Is this _only_ for receiving input from a socket via a handle, or can I use it to write to as well - some examples on the web use code that writes directly to the $handle ex) "print $first "hello"; ... is this correct form?

If I need to send a message across the socket, should I instead use print $socket "my message"; ? Should $handle = $socket->accept; only be used when listening on a socket?