my $input = "CAT,DOG,BIRD,COW";
$input =~ /\A\w*(?:,\w*)*\z/ or die "invalid input format";
my $str = join ',', map { "'$_'" } split /,/, $input;

##</code><code>##

my $str = join ',', map { $dbh->quote($_,'VARCHAR') } @values;
my $str = join ',', map { "'$_'"                    } @values;