#!/usr/bin/perl -l

use warnings;
use strict;
use Socket;

my $sock_fh;

BEGIN {
    my $port = 987654; # any constant specific to this app
    socket($sock_fh, PF_INET, SOCK_STREAM, getprotobyname('tcp'))
	or die "socket: $!";
    bind($sock_fh, sockaddr_in($port, INADDR_LOOPBACK))
	or (warn "another process is already running ($!)\n"
	    and exit(1));
}

# BEGIN: where you do your work
print "send interupt to kill process";
while (<>) {
    sleep 1; 
}
# END: where you do your work

END {
    $sock_fh->close() if defined $sock_fh;
}