I don't know why you are getting no substitutions, but if I were doing this, instead of storing words in the DB, I'd store regexp strings (e.g. '\bfudge(?:d up)?\b') that I would then convert to regexps using qr; e.g.:

   while(my $ref = $sth->fetchrow_arrayref) 
   {
       push @sw_regexps, qr($ref->[0]);
   }
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Also, you probably want the /g modifier in that s///