sub tee {
  my $file = shift;

  open  FILE, "> $file"
    or die "Cannot open($file) for writing: $!\n";

  return if my $pid = open STDOUT, "|-";

  die "Cannot fork STDOUT: $!\n" unless defined $pid;

  print FILE while <STDIN>; 
  close FILE;

  exit;
}

tee('/some/file');

while ( <> ) {
  print;
}
close STDOUT; # be nice to tee's