Quote from the perlre man page:

A word boundary ("\b") is a spot between two characters that has a "\w" on one side of it and a "\W" on the other side of it (in either order)

So, the \b will only match in your case if it is followed by a word character (alphanumeric plus "_"), in your case it's probably followed by whitespace, so it doesn't match.
hth,
snowcrash