##################
# Server:

open my $msgs, "| tail -f data.txt" or die "ouch: $!";
while (<$msgs>) {
 # do something with this line of data
}


##################
# Client:
use Fcntl ':flock';

while (<STDIN>) {
 chomp;
 open my $msgs, '>>', 'data.txt' or die "ouch: $!";
 flock $msgs, LOCK_EX;
 print( $msgs, ($_ . "\n") );
 close $msgs;
}