As another data point, I did some tests with my own simple "echo" program. I was able to confirm that the first problem, "hangs with blank output" was caused by output buffering. I was unable to find any way to get an EOF recognized in "echo2.pl" for reasons that I don't understand. closing $writer didn't do it. So this still hangs, just further along in the process. I was hoping to find some way to get "echo2.pl" to exit, but that didn't happen.

The pipe will have a finite size and if you try to cram too much stuff into it via a blocking write, then it can "fillup" and you can deadlock while trying to cram even more into the pipe. But that's not the problem in this simple example.

I think BrowserUk's suggestion is a good one. Although it is not completely clear to me why this fails. And of course, it is seldom that we have control over buffering settings the other process.

use strict;
use warnings;
use IPC::Open2;

$|=1;
$SIG{PIPE} = sub { print STDERR "SIGPIPE received\n"; exit; };
my $pid = open2(my $reader, my $writer, "perl echo2.pl");
print "pid=$pid\n";

print $writer "$_\n" for (qw(first second third ));
print "Data written\n";
print $writer chr 4; #try manually send EOF - didn't work!
close $writer;       #reader still "hangs"

print "Reading from pipe:\n";
while( my $in=<$reader>) {
    print  "Received: ".$in."\n";
}
close $reader;
print "No more data\n";
waitpid($pid, 0);
print "Finished\n";

=echo2.pl
#!/usr/bin/perl -w
use strict;

$|++;
print "first test line\n";
while (<STDIN>){print;}
=cut

__END__
C:\TEMP>perl ipc_open2.pl
pid=5688
first
second
third
Data written
Reading from pipe:
Received: first test line

Received: first

Received: second

Received: third
####now hung up in reader ###
#### for some reason it doesn't see that input pipe
#### closed
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Update: Things are a lot easier if this is like a "one line in", "one line out" interface. Or if the response has a "end of transmission" flag, something like blank line means "end of transmission" so that we can quit trying to read more lines that will not be forthcoming.

use strict;
use warnings;
use IPC::Open2;

$|=1;   # this is for main program
        # $writer is alread unbuffered by open2
my $pid = open2(my $reader, my $writer, "perl echo2.pl");
print "pid=$pid\n";

for ( qw(first second third) )
{
   print $writer "$_\n";   # one line in, one line out
   my $result = <$reader>;
   print $result;
}

=heading #### echo2.pl #####
#!/usr/bin/perl -w
use strict;

$|=1;
while (my $in=<STDIN>){print $in;}
print STDERR "out of loop\n";  #never gets here!
=cut  #### echo2.pl #####

__END__
C:\TEMP>perl ipc_open2.pl
pid=3244
first
second
third
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