Here's an 'OO-style' example (including a server part, so it can be run standalone):

#!/usr/bin/perl -w
use strict;
use IO::Socket;

my $pid = fork();
unless ($pid) {
    # --- server
    my $server = IO::Socket::INET->new(
        #LocalAddr => 'localhost',
        LocalPort => 12345,
        Proto     => "tcp",
        Listen    => SOMAXCONN, 
        Reuse     => 1,
    ) or die "couldn't create listening socket: $!";

    print "server listening...\n";
    
    while (1) {
        my $client = $server->accept();
        print "client connected\n";
        while (<$client>) {
            if (/^foo/) {
                $client->print("bar");       # 3 bytes (dummy)
                $client->print("0000001f");  # 8 bytes length as hex s
+tring
                $client->print("X" x 0x1f);  # data
                last;
            }
        }
        $client->close();
        print "connection closed\n";
    }
    exit;

} else {  # --- client
    sleep 1;

    my $conn = MyConnection->new('localhost', 12345);
    $conn->WriteInfo("foo\n");
    my $data = $conn->ReadData();
    print "data: '$data'\n";
}

END {
    kill 15, $pid;
    print "server shut down\n";
}


package MyConnection;
use base "IO::Socket::INET";

sub new {
    my $class = shift;
    my ($addr, $port) = @_;

    my $socket = IO::Socket::INET->new(
        Proto    => "tcp",
        PeerAddr => $addr,
        PeerPort => $port,
    ) or die "couldn't connect: $!";

    $socket->autoflush();
    return bless $socket, $class;
}

sub WriteInfo {
    my $sock = shift;
    my $info = shift;
    print "client sending request\n";
    $sock->print($info);
}

sub ReadData {
    my $sock = shift;
    $sock->read(my $dummy, 3);
    $sock->read(my $len_hex, 8);  
    my $len = hex($len_hex);
    print "client reading response ($len bytes)\n";
    $sock->read(my $data, $len);
    return $data;
}
[download]

$ ./892650.pl 
server listening...
client connected
client sending request
client reading response (31 bytes)
data: 'XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX'
connection closed
server shut down
[download]

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