Says masem:

{ $topic =~ s/([^/W])$_([$/W])/$1$2/g; }

There are several errors here. You have /W where you seem to have meant \W. You have ^ inside a character class, where it means to negate the class, but you seem to have wanted it outside, so that it means beginning-of-string. You have $ inside the other character class, so you are interpolating $/ into the regex, and you seem to have wanted the $ in a place where it would mean end-of-string. I think you might have meant:

  s/(^|\W)$_(\W|$)/$1$2/g
[download]

But the thing I suggested with \b is simpler anyway.

--
Mark Dominus
Perl Paraphernalia