Why are you globally enabling "slurp mode" (with $/=undef) ?

This disable the "readline mode" when you use $msg=<STDIN>, so Perl will wait forever until it gets a End-Of-File. It means, that entering your message and hitting ENTER will not give you back the message in $msg, so you'll have to type ^D (CTRL+D) in order to process your entry.

If you just want to "slurp" your CONFIGFILE (and nothing else), you should first open CONFIGFILE, then locally disable $/ :

open CONFIGFILE, "config.txt" or die $!;
{
    local $/;
    $/ = undef;
    $DATA=<CONFIGFILE>;
}
print $DATA;
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That way, $/ remains in default "line mode" for your read from <STDIN> (see perlvar for details on scope with $/).

Beside this "slurp mode" problem, your code seems to run well on my machine with "nc -u localhost 1234" as a client.