Fear not, there are many way to accomplish what you want without resorting to magic, many of which are well documented here in the Monastery:

• how do i redirect STDOUT, STDIN, or STDERR to a FILE?
• Super search also gives many threads on related topics
• also google
• Hook::Output::File / you can also search CPAN for modules that do what you want

use Eight::Ball;

open my $file, '>', $filename or die "unable to open $filename";
open my $oldout, '>&', STDOUT or die "unable to dup STDOUT";
my $pid = open STDOUT, '|-';
unless ($pid) {
  my $ball = Eight::Ball->new();
  while (<>) {
    if ($ball->tell_me("should I redirect the line?")) {
      print $file $_;
    }
    else {
      print $oldout $_;
    }
  }
  exit(0);
}

# your code goes here!
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