my @blacklist = ('evil', 'bad', 'wrong');
my $a = "this string contains no blacklisted tokens";
my $b = "this string is evil and wrong";

# The regex should express the blacklist is such a way that
# it will match on any string which DOES NOT contain any of
# the tokens in the blacklist, and it will fail to match on
# any string which DOES contain tokens from the blacklist.

my $regex = '?????';

if (($a =~ m/$regex/) && !($b =~ m/$regex/)) {
   # SUCCESS
}