Perl & C code follow:
----------------------------

##</code><code>##

HelloWorld.pl
#!/usr/bin/perl -w
# HelloWorld.pl   pipe2 - bidirectional communication using socketpair
#   "the best ones always go both ways"


use Socket ;
use IO::Handle;     # thousands of lines just for autoflush :-(

# We say AF_UNIX because although *_LOCAL is the
# POSIX 1003.1g form of the constant, many machines
# still don't have it.
socketpair(CHILD, PARENT, AF_UNIX, SOCK_STREAM, PF_UNSPEC)
                                or  die "socketpair: $!";


CHILD->autoflush(1);
PARENT->autoflush(1);


if ($pid = fork) { # parent
#exit ;
   close PARENT;
   do {
      print CHILD "Parent Pid $$ is sending this\n";
      print "Parent Pid $$ just read this: `$line`\n";
   } while( chomp($line = <CHILD>) )  ;
   close CHILD;
   waitpid($pid,0);
} else {           # child
   die "cannot fork: $!" unless defined $pid;
   close CHILD;
   while( chomp($line = <PARENT>)) {
      print "Child Pid $$ just read this: `$line`\n";
      print PARENT "Child Pid $$ is sending this\n";
   } 
   close PARENT;
   exit;
}


----------------------------------------------------------
spair.c
#include <stdio.h>
#include <stdlib.h>
#include <ctype.h>
#include <errno.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/socket.h>

char * strupr( char * string ) ;          /* Change string to uppercase */

int main(void)
{
    int sv[2]; /* the pair of socket descriptors */
    char buf[1024];  /* for data exchange between processes */
    int i ;    /* counter */
    char *ch =  "a" ;

    char * mydata[] = {
                       "alfa",
                       "bravo",
                       "charlie",
                       "delta",
                       "echo"
                      } ;

    if (socketpair(AF_UNIX, SOCK_STREAM, 0, sv) == -1) {
        perror("socketpair");
        exit(1);
    }

    if (!fork()) {  /* child */ 
/*execlp ("subsn.pl", "subsn.pl", 0);*/ /* Fire the substitution module */ 
execlp ("HelloWorld.pl", "HelloWorld.pl", 0); /* Fire the substitution module */ 
        for ( i = 0 ; i < 5 ; i ++ ) {
           read(sv[1], &buf, sizeof( buf ) );
           printf("child:  read [%s]\n", buf);
           /*buf = toupper(buf);*/  /* make it uppercase */
           strupr( buf ) ;  /* make it uppercase */
           write(sv[1], &buf, sizeof(buf));
           printf("child:  sent [%s]\n", buf);
        }

    } else { /* parent */
        for ( i = 0 ; i < 5 ; i ++ ) {
           strcpy( buf, mydata[ i ] ) ; /* copy data to buffer */
           write(sv[0], buf, sizeof( buf));
           printf("parent: sent [%s]\n", buf );
           read(sv[0], &buf, sizeof(buf));
           printf("parent: read [%s]\n\n", buf);
        }
        wait(NULL); /* wait for child to die */
    }

    return 0;
}

/*
** strupr : Change string to uppercase
*/
char * strupr( char * string )
{
   char * original = string ;

   while ( * string ) {
      * string = _toupper( *string ) ;
      string++ ;
   }
   return ( original ) ;
}