Rework change 5787 in which support for taint checking of arguments
passed to P4::Run() was introduced. It turns out Scalar::Util, on
which this is based isn't distributed with Perl 5.6, so we can't
rely on it. This change simply disables the taint checking for
Perl 5.6 (no-one's using it because it only ever worked for Perl 5.8
users).