sub client1 {
    my $fno = shift;
    open my $client, '+<&', $fno or die;
    while( <$client> {
         if( /... / {
             print $client ...;
         }
         elsif( /.../ ) {
             print $client ...;
         else {
             print $client ...;
         }
    }
}
sub client2 { ... }
sub client3 { ... }
sub client4 { ... }
sub client5 { ... }

my %lookup = (
    'xxx.xxx.xxx.xxx' => \&client1,
    'yyy.yyy.yyy.yyy' => \&client2,
    ...
);

my $lsn = IO::Socket::INET->new( Listen => 5, LocalPort => $port );

while( my $client = $lst->accept ) {
    my $addr = $client->peerhost;
    thread->create( $lookup{ $addr }, fileno( $client ) )->detach;
}