Probably it's because the data you're trying to match is full of slashes, which you're also using as delimiters for your match. Either try different delimiters (e.g., "!"), or look into the quote-endquote metacharacters \Q and \E.

Update: As Emily Litella used to say, "Never mind." My answer might have been appropriate if you were trying to do a substitution such as s/$_/stuff/g; but that it wasn't your question. Sorry.

BCE
--Your punctuation skills are insufficient!