Here's a test to make sure that the data contains nothing
but "word" characters (alphabetics, numerics, and under-
scores), a hyphen, an at sign, or a dot.

    if ($data =~ /^([-\@\w.]+)$/) {
        $data = $1;                     # $data now untainted
    } else {
        die "Bad data in $data";        # log this somewhere
    }

##</code><code>##

#!/usr/bin/perl -wT

package Parent;
1;

package Child;
@ISA = qw(Parent);
1;

package main;

%ENV = ();

use strict;

our $isa;

$|++;
print "Type in something: ";
$isa = <STDIN>;

$isa = "/bin/ls";
print "(qx)  ", (eval { qx($isa); 1 } ? "Untainted" : "Tainted"), "\n";

$isa = "Child";
print "(isa) ", ($isa->isa("Parent") ? "Untainted" : "Tainted"), "\n";

##</code><code>##

Type in something: blah
(qx)  Untainted
(isa) Tainted