They would listen to a specified port (23 or any other port, you could telnet hostname XXX to connect to port XXX) and run your script redirecting stdin/stdout to that connection.. Your script can be as simple as you want,dumb example:

#!/usr/bin/perl -w

use strict;

$|=1;

print "What is your name?";
my $name=<>;
chomp $name;

print "Hello, $name, nice to meet you\n\n";
my $choice='';

while($choice ne 'q')
{
  print "a. Print 1+1\n";
  print "b. Print Hi\n";
  print "q. Quit!\n";
  $choice=<>;
  chomp $choice;
  if ($choice eq 'a') {print "1+1=2\n";}
  if ($choice eq 'b') {print "Hi $name !\n";}
}
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How to use xinetd on linux/unix:

remeber to disable output buffering $|=1

Simple: create a file /etc/xinetd.d/yourscriptname with

service yourscriptname
{
        disable = no
        socket_type     = stream
        wait            = no
        user            = root
        server          = /path/to/your/script
}
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also add to your /etc/services:

yourscriptname    portno/tcp
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There are inetd ports to windows...