I have just one question: how does this compare to using a magic open?

A magic open is a really cool Perl feature that let you... set a filter on a file! Just like this:

#!/bin/perl -w

use strict;

set_filter();                     # sets the post-processing filter
print "main processing is here\n";
print "and continues here\n";

exit;

BEGIN
  { my $i=1;

  sub set_filter
    { return if my $pid= open( STDOUT, "|-");    # magic open
      die "cannot fork: $!" unless defined $pid;

     while( <STDIN>)
        { printf "line %03d: %s", $i++, $_; } 
      exit;
    }
  }
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