\b is a zero-width assertion, so I don't think it matters one way or the other...
/\b($patt)\b/ wont behave any differently than /(\b$patt\b)/, though the first one looks clearer to my eyes.

p.s. When posting code to perlmonks, you should surrond your code with CODE tags.

<CODE>my $perl = 'stuff';</CODE>

-Blake