There is regular expression support in MySQL that allows for queries like this:

sub detect_profanity {
    my $str = shift;
    my $sth = $dbh->prepare (qq{ SELECT * FROM `forbidden_usernames` W
+HERE `f_user` REGEXP ? });
    $sth->execute($str);
    my $row = $sth->fetchrow_hashref();
    $sth->finish();

    return "bad" if $row;
}
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You can also use several special characters/constructs to limit the matches. See the MySQL docs for more info.