my $iLength = length('AU: Written by:');

# read lines one at a line instead of slurping into an array
# this is more memory friendly
while (my $line = <INPUT>) {

  # single out lines beginning with 'AU' for special processing
  if ($line =~ /^AU:/) {
     
     # insert "Written by"
     # Note: substr on the left means
     #   replace the zero-length string at position 3
     #   in other words: insert something at position 3
     substr($line, 3,0) = ' Written by:';

     # capitalize the first letter of every word
     # Note: substr on the left means
     #   make substitutions in the part of the string after
     #   "AU: written by" 
     substr($line, $iLength) =~ s/\s(\w+)/ \u$1/g;
  }
  print $line;
}