As @brian is mentioning in the comments the currently accepted answer by @piCookie is wrong!
$_="what's the wrong answer?";
s/\b(\w)/\U$1/g
print;
This will print “What’S The Wrong Answer?” notice the wrongly capitalized S
As the FAQ says you are probably better off using
s/([\w']+)/\u\L$1/g
or Text::Autoformat