use strict;
use warnings;
my $ariths = quotemeta '()+</*><-';
while ( <DATA> ) {
   s/(?:(?:(?<=[$ariths])BAD)|^BAD)(?=[$ariths])/HELL/g;
   print;
}

__DATA__
+BAD        # No
BAD/       # Yes
BAD)       # Yes
(BAD<)     # Yes
BAD         # No
BADGER      # No
BADGER+     # No
+SINBAD+    # No      
BAD (BAD+  # No then Yes