Indeed.

Though perhaps what the questioner actually wants is to run the script at the server end, but display the script's output at the client end? (And is just not expressing that especially clearly.) That is, after all, a fairly common desire.

In which case, it's something along these lines...

my $output = `perl the-script.pl`;
print CLIENT $output;
[download]

I tried it, and added the lines:

my $output = `perl the-script.pl`;
print CLIENT $output;
[download]

But it just prints "smile from the server" and doesnt run the perl program. :-/

Here is the total code:

#! /usr/bin/perl -w
# server0.pl
#--------------------

#use strict;
use Socket;

# use port 7890 as default
my $port = shift || 7890;
my $proto = getprotobyname('tcp');

# create a socket, make it reusable
socket(SERVER, PF_INET, SOCK_STREAM, $proto) or die "socket: $!";
setsockopt(SERVER, SOL_SOCKET, SO_REUSEADDR, 1) or die "setsock: $!";

# grab a port on this machine
my $paddr = sockaddr_in($port, INADDR_ANY);

# bind to a port, then listen
bind(SERVER, $paddr) or die "bind: $!";
listen(SERVER, SOMAXCONN) or die "listen: $!";
print "SERVER started on port $port ";

# accepting a connection
my $client_addr;
while ($client_addr = accept(CLIENT, SERVER))
{
# find out who connected
my ($client_port, $client_ip) = sockaddr_in($client_addr);
my $client_ipnum = inet_ntoa($client_ip);
my $client_host = gethostbyaddr($client_ip, AF_INET);
# print who has connected
print "got a connection from: $client_host","[$client_ipnum] ";
# send them a message, close connection
    print CLIENT "Smile from the server\n";
#    open CLIENT, ("system ("perl io-test-1.pl");");
#    open (CLIENT, "system ("perl io-test-1.pl")");
#    open CLIENT, "do `perl io-test-1.pl`";
#my $status = CLIENT, system("perl io-test-1.pl");
#CLIENT $status = CLIENT, system("perl io-test-1.pl");
    my $output = `perl io-test-1.pl`;
    print CLIENT $output;
    close CLIENT;
}
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So, what does io-test-1.pl do? Does it actually generate output? Does it compile? Does it even exist?

PS: commenting out your use strict; is a pretty bad idea. Ignoring problems doesn't solve them...