You should be using Filter::Handle, no doubt. But if you want to do it yourself (say as a quick and dirty hack), it's actually fairly easy. The perldoc perltie manpage has the details.

Of course, you're still probably best off dividing it into 2 parts, a file containing the module IO::SubHandle: (properly located) and the main program.

#!/usr/local/bin/perl -w

package IO::SubHandle;

use strict;

sub TIEHANDLE {
  my $class = shift;
  local *FH = shift;
  my $sub = shift;

  bless { handle => *FH{IO}, routine => $sub }, $class;
}

sub PRINT {
  my $self = shift;
  my $fh = $self->{handle};
  print $fh ($self->{routine}->(@_));
}

# Not clear how to handle this!
sub PRINTF {
  my $self = shift;
  my $fmt = shift;
  $self->PRINT(sprintf($fmt => @_));
}


package main;

use IO::SubHandle;

sub filterText {
  ("Filter [@{[scalar @_]}]: ", @_);
}

print "This is your normal STDOUT\n";

tie *STDOUT, 'IO::SubHandle' => *STDOUT, \&filterText;

print "This is your filtered STDOUT\n";
print STDOUT "This also gets filtered...\n";
print STDERR "STDERR is safe...\n";
printf STDOUT "PRINTF(pi = \%g) tied too (but #args different)\n", 3.1
+4159;
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Depending on what operations you perform, you may need to tie some more methods! Note also how the printf tie probably isn't implemented "correctly"; you'll need to define precisely what it should do, then you can do it.