+------------------------+----------------------+
| Parent                 | Child                |
+------------------------+----------------------+
| - open3(...);          |                      |
| - <OUT>                | - ...                |   |
|   - Waits for child to | - print STDERR 1027; |   |
|     write to STDOUT or | - print STDERR 1028; |   |
|     for child to end.  | - print STDERR 1029; |   T
|                        | - print STDERR 1030; |   i
|                        | - print STDERR 1031; |   m
|                        |   - Pipe full.       |   e
|                        |     Waits for parent |   |
|                        |     to empty it.     |   |
|                        |                      |   v
| Still waiting...       | Still waiting...     |
+------------------------+----------------------+

##</code><code>##

my %lookup = (
   \*OUT => [ \*OUT, 'STDOUT', '' ],
   \*ERR => [ \*ERR, 'STDERR', '' ],
);

MAIN_LOOP:
for (;;) {
   my @r = $r_sel->can_read();

   foreach my $fh (@r) {
      last MAIN_LOOP if eof($fh);

      my $buf_ptr = $lookup{$fh}[2];
      1 while read($fh, $$buf_ptr, $BLOCK_SIZE, length($$buf_ptr));

      print("$lookup{$fh}[1]: $1")
         while $$buf_ptr =~ s/^(.*\n)//;
   }
}

foreach my $fh (keys %lookup) {
   print("$lookup{$fh}[1]: $lookup{$fh}[2]\n")
      if length $lookup{$fh}[2];
}