if($name ne $user) {

##</code><code>##

    while(<USERNAME>)                     # reads first entry
        {
        my @newusername = <USERNAME>;     # reads remaining entries
    foreach $name (@newusername)
        ...

##</code><code>##

...
my $found;
while (my $name = <USERNAME>) {
    if ($name eq $user) {
        $found = 1;
        last;
    }
}
unless ($found) {
    print $client_socket "ERR\n";
    ...
    exit;
}
print $client_socket "ACK\n";