I am not sure about your question.
If the problem is buffering be sure you have set them to flush:

use IO::Handle;

$F = new IO::Handle;
$F->autoflush(1);
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If what you want is to filter the output then create a filter by forking again like in this example

$ cat -n openself
 1  #!/usr/local/bin/perl -w
 2  use strict;
 3  use utf8;
 4  binmode(STDOUT, ':utf8');
 5  binmode(STDIN,  ':utf8');
 6
 7  print "Father: $$\n";
 8
 9  my $pid = open(STDOUT, "|-");
10  die "cannot fork: $!" unless defined $pid;
11
12  # Child filters father's output
13  filterstdout() unless $pid;
14
15  # Father
16  print " <- This is the PID of the child\n";
17  while (<>) {
18      print
19  }
20
21  sub filterstdout {
22    while (<STDIN>) {
23      tr/áéíóúñ€/aeioun$/;
24      print "$$: $_";
25    }
26    exit;
27  }
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Hope it helps

Casiano