Here's my version, with some important revisions

• shebang line with -w
• use strict; to catch stupid errors
• IO::Handle provides a very nice getline() method, so I used that instead of trying to interpolate what your read_line does. When doing IO, almost anything you write will be already done better and faster by someone else - if I find myself wrapping sysread or such like, I only do so because I know that is what is required. If I'm guessing, I look for a builtin.
• I made it a little more obvious who was reading and writing in the debugs. Probably better if you write to STDERR, as that isn't buffered by default.

#!/usr/bin/perl -w

use strict;

use IO::Select;
use IO::Pipe;
use IO::Handle;

my $debug = 1;

my @connect = ("3","2","1");

my %msgs;
@msgs{@connect} = qw/hooray hip hip/;

my $select_ch = IO::Select->new();

foreach my $msg (@connect) {
    # for each write, fork a child
    if ($msg) {
        # fork and open child for reading
        my $pipe = IO::Pipe->new();

        if (my $pid = fork() ) {
        #parent
        my $fh = $pipe->reader();
        $debug && print "parent $$ forked $pid\n";
        $fh->blocking(0);    # set non-blocking I/O
        $debug && print "parent adding readable filehandle ($fh)\n";
        $select_ch->add($fh);
        } else {
        #child
        my $childhandle = $pipe->writer();
        my $mt = $msgs{$msg};
        sleep $msg;
        print STDERR "$$ child $msg writing to pipe $msg, response $mt
+\n";
        print $childhandle "child $msg, response $mt";
        exit 0;        #kill the child process
        }
    }
}                # done with processing this read

# here we read for responses from the filehandles
my @readyfiles;            #array for select to populate
while ( @readyfiles = $select_ch->can_read() ) {
    foreach my $rh (@readyfiles) {
    # read from filehandle, send back string

    print "$$ calling IO::Handle::getline\n";

    my $child_resp_string = $rh->getline();

    $debug && print "$$ read |$child_resp_string|\n";

    # only one read per handle, so close
    $debug && print "$$ removing @{[ref $rh]}\n";
    $select_ch->remove($rh);
    $rh->close;

    $debug && print "$$ @{[$select_ch->count()]} handles left to read\
+n";
    }
}
print "exiting...\n";
exit 0;
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Platforms
Linux itdevtst 2.4.20-8 #1 Thu Mar 13 17:54:28 EST 2003 i686 i686 i386 GNU/Linux
SunOS robin 5.8 Generic sun4u sparc SUNW,Ultra-60

Perl versions
This is perl, v5.8.2 built for i686-linux
This is perl, v5.8.1 built for sun4-solaris

Output

le99007@robin:~/src/perl> ./pipe.pl 
parent 7146 forked 7147
parent adding readable filehandle (IO::Pipe::End=GLOB(0x195708))
parent 7146 forked 7148
parent adding readable filehandle (IO::Pipe::End=GLOB(0x1957a4))
parent 7146 forked 7149
parent adding readable filehandle (IO::Pipe::End=GLOB(0x151eb0))
7149 child 1 writing to pipe 1, response hip
7146 calling IO::Handle::getline
7146 read |child 1, response hip|
7146 removing IO::Pipe::End
7146 2 handles left to read
7148 child 2 writing to pipe 2, response hip
7146 calling IO::Handle::getline
7146 read |child 2, response hip|
7146 removing IO::Pipe::End
7146 1 handles left to read
7147 child 3 writing to pipe 3, response hooray
7146 calling IO::Handle::getline
7146 read |child 3, response hooray|
7146 removing IO::Pipe::End
7146 0 handles left to read
exiting...
[download]

Same result on the Linux box

Thanks for the help! Strangely enough, when I copied the code I had posted (which was an edited version of my full test code), it worked correctly. Still not sure why my first version was failing.

I had avoided $pipe->getline($fh) because the documentation said it behaved like <$fh> and I was under the impression that select (even IO::Select) and <> were incompatible. Now that I think about it, behaving like <> and actually being <> are two different things. So, now I've converted my production code to use IO::Pipe and now have a whole new set of problems :)

Thanks again for your assitance.