DBD::Pg and taint (-T) mode

Hi.  I have conflicting information on DBD::Pg's support for taint mode, and
would like some clarification from you if possible.

Is there any way to tell DBD::Pg (or DBI) to raise an exception when prepare
is invoked with a tainted SQL string?

I thought this would be the default behavior, and was surprised when I ran
the demo script below.

This script requires four command-line arguments: the name of  a database,
the name of a table in that database, the name of an integer-type column in
that table, and some integer.  E.g., a run may look like this:

$ perl -T demo_script.pl mydb mytable mycolum 42
1
1
1
1
1
1

NB: you will need to modify the user and password parameters in the call to
DBI->connect.

The important thing to note is that the connect, prepare, and execute
methods all receive tainted arguments, but run without any problem.
 Furthermore, the subsequent fetchall_arrayref also runs without any
problem.

Many thanks in advance.  Perl code follows.

~K


#!/usr/bin/perl

use strict;
use warnings FATAL => 'all';

use DBI;

my $dbname = shift;
my $tablename = shift;
my $colname = shift;
my $id = shift;
my $sql = qq(SELECT * FROM "$tablename" WHERE "$colname" = \$1;);
my $connection_string = "dbi:Pg:dbname=$dbname";

# when this script is run under -T, the output from all the following
# print statements is 1; if the script is *not* run under -T, then
# they are all 0.
print +(is_tainted($dbname) ? 1 : 0), "\n";
print +(is_tainted($tablename) ? 1 : 0), "\n";
print +(is_tainted($colname) ? 1 : 0), "\n";
print +(is_tainted($id) ? 1 : 0), "\n";
print +(is_tainted($connection_string) ? 1 : 0), "\n";
print +(is_tainted($sql) ? 1 : 0), "\n";

my $dbh = DBI->connect($connection_string,
                       "kynn", undef,
                       +{
                          RaiseError => 1,
                          PrintError => 0,
                          PrintWarn  => 0,
                        });

my $sth = $dbh->prepare($sql);
$sth->execute($id);
my $fetched = $sth->fetchall_arrayref;

sub is_tainted {
  # this sub is adapted from Programming Perl, 3rd ed., p. 561
  my $arg = shift;
  my $empty = do {
    no warnings 'uninitialized';
    substr($arg, 0, 0);
  };
  local $@;
  eval { eval "# $empty" };
  return length($@) != 0;
}

~K