# Capitalizes all words - this will turn 
#      don't feed ducks
# into 
#      Don'T Feed Ducks
#          ^--!

$sentence =~ s/\b(\w)/\U$1/g;


# a better solution, taking care of such situations:
$sentence =~ s/(
                 (^\w)    #at the beginning of the line
                   |      # or
                 (\s\w)   #preceded by whitespace
               )
              /\U$1/xg;
$sentence =~ /([\w']+)/\u\L$1/g;