Rather than TCP; use UDP.

In your perl program you only need this to receive the notification:

#! perl -slw
use strict;
use IO::Socket;

socket( SOCKET, PF_INET, SOCK_DGRAM, getprotobyname('udp') ) or die "s
+ocket: $!";
bind( SOCKET, sockaddr_in( 54321, inet_aton( 'localhost' ) ) ) or die 
+$!;
my $input;

while( 1 ) {
    my $addr = recv( SOCKET, $input, 1024, 0 ) or die $!;
    my( $port, $ipbin ) = sockaddr_in( $addr );
    printf "Got '%s' from  ip:%s port:%d\n", $input, inet_ntoa($ipbin)
+, $port;
}
[download]

In your PHP program, you can use this to send the '1' or '2' when required.

The Perl script above will block at the recv until it receives the datagram from your PHP program.