So you want one process to pay attention to what a completely independent process is dong while the 'watching process' keeps its back turned and eyes closed so as to speak. As the processes are independent you will have to poll/signal somehow or other. Your options would include

If your on Windows take a look at Win32::Semaphore for a simple IPC signalling mechanism if you can modify the other process, or Win32::ChangeNotify to have the OS inform you when the file is modifed regardless of who modifies it.

The easiest way I can think of is to use the Msg.pm module from Advanced Perl Programming (it's in the Networking directory).

In your running process you would put the listener code (ok, so it's more of a server), i.e.:

use Msg;
use strict;
my $host = 'localhost';
my $port = 8080;
Msg->new_server($host, $port, \&login_proc);
print "Server created. Waiting for events";
Msg->event_loop();
#---------------------------------------------------------------
sub login_proc {
    # Unconditionally accept an incoming connection request
    return \&rcvd_msg_from_client;
}


sub rcvd_msg_from_client {
    my ($conn, $msg, $err) = @_;
    if (defined $msg) {
        print "$msg\n";
    }
}
[download]

From the text, "The script calls new_server to create a listening socket (the program's network address) and then calls event_loop, an event dispatcher, which is a thin wrapper over select."

And the client code that would send a message to your running process (server):

use Msg;
$conn = Msg->connect('localhost', 8080);
die "Error: Could not connect\n" unless $conn;
$conn->send_now("Message $i");
($msg, $err) = $conn->rcv_now();
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Use the server and client scripts from the book's example code because the code here is for illustrative purposes.

--
hiseldl
What time is it? It's Camel Time!

# could do stat()[9] and not use File::stat
use File::stat qw(:FIELDS);

$cfgfile = '/my/config/variables.file';

stat($cfgfile) or die "No $cfgfile: $!";
my $last_mtime = $st_mtime;
parse_config($cfgfile);
#time passes
#enter loop of things to do and events to check
stat($cfgfile) or die "No $cfgfile: $!";
if ($last_mtime != $st_mtime) {
  $last_mtime = $st_mtime;
  parse_config($cfgfile);
}
# sleep(N) unless something else to do
[download]

Dingus