Everyone has advised you to use a regex so far. However, when you know the position in the string to check for, substr is often a better method, as it is clearer and potentially has a lower overhead.

$new .= '/' unless substr $new, -1 eq '/';
[download]

Light a man a fire, he's warm for a day. Catch a man on fire, and he's warm for the rest of his life. - Terry Pratchet