Two common misconceptions are that \b is a synonym for \s+ and that it's the edge between whitespace characters and non-whitespace characters. Neither is correct. \b is the place between a \w character and a \W character (that is, \b is the edge of a "word"). It's a zero-width assertion, just like ^, $, and all the other anchors, so it doesn't consume any characters. perlre describes the behavior of all the regex metacharacters.
Here are examples of the incorrect application of \b, with fixes:
"two words" =~ /(\w+)\b(\w+)/; # WRONG
"two words" =~ /(\w+)\s+(\w+)/; # right " =matchless= text" =~ /\b=(\w+)=\b/; # WRONG
" =matchless= text" =~ /=(\w+)=/; # rightAlthough they may not do what you thought they did, \b and \B
can still be quite useful. For an example of the correct use of
\b, see the example of matching duplicate words over multiple
lines.
An example of using \B is the pattern \Bis\B. This will find occurrences of "is" on the insides of words only, as in "thistle", but not "this" or "island".