#!/usr/bin/perl
use warnings;
use strict;
use IO::Handle;

$|++;
pipe( PARENT_READER, CHILD_WRITER );
pipe( CHILD_READER,  PARENT_WRITER );
PARENT_WRITER->autoflush(1);
CHILD_WRITER->autoflush(1);

my $pid;
if ( $pid = fork() ) {
    close(CHILD_READER);
    close(CHILD_WRITER);
    my $buffer;
    print PARENT_WRITER 1;
    while (1) {
        sysread( PARENT_READER, $buffer, 100 );
        print "parent revd: $buffer, and reply with ", $buffer + 1, "\
+n";
        sleep(1);
        print PARENT_WRITER $buffer + 1;
    }
}
else {
    close(PARENT_READER);
    close(PARENT_WRITER);
    my $buffer;
    while (1) {
        sysread( CHILD_READER, $buffer, 1000 );
        print "child revd: $buffer, and reply with ", $buffer + 1, "\n
+";
        sleep(1);
        print CHILD_WRITER $buffer + 1;
    }
}
[download]

but this simpler example may be more useful to you.

#!/usr/bin/perl
use strict;
use warnings;

my $pid = open(CHILD, "-|");

if ($pid)
{
   # parent
   print "parent got:\n";
   print while(<CHILD>);
   close CHILD; # this waits on child
}
elsif ($pid == 0)
{
   # child
   print "kid here!\n";
   exec '/bin/date' or die "Can't exec date $!\n";
}
else
{
   die "fork error: $!\n";
}
[download]

A little starting point:

#!/usr/bin/perl
use strict;
use warnings;

print "Starting external program. Waiting for input.\n";
open my $fh, "-|", "while true; do date; sleep 1; done";
my @lines;
while(defined(my $line = <$fh>)) {
    chomp $line;
    push @lines, $line;
    if(@lines >= 20) {
        my $howmuch = @lines;
        print "Read '$howmuch' lines from external program.\n";
        print "Stats are...calc calc calc.\n";
        print "The following lines where read:\n";
        print map { "$_\n"; } splice @lines;
        print "Going to sleep.\n";
        sleep 20;
    }
}
[download]

Looking at the docs of the used commands should push you forward. Search here helps also (reading from external program, capturing from external program).

Best regards
McA

Hi, Thanks this has helped. I am now able to generate my files once it has the 20 lines

I'm happy to hear that. Thanks for the feedback.