From PerlNet

G'day Chromatic,

Today you wondered if Modern::Perl should enable taint mode.  Unfortunately,
my response wouldn't fit into 140 chars, so you're getting it via e-mail.
Feel free to republish or publicly discuss this mail if you wish.

I feel that Modern::Perl should enable all the modern features and pragmas
that are desirable in modern Perl programs, and (ideally) should disable or
remove features that are undesirable.  Right now all my programs start with:

	use strict;
	use warnings;
	use 5.010;
	use autodie;

Currently I can reduce this to:

	use Modern::Perl;
	use autodie;

and this is a great improvement.  If I wanted to get really lazy, I'd
combine the two into Modern::Paul.   ;) 

So currently, Modern::Perl makes my life easier.  However if it comes to
enabling taint mode, that starts to be a bit more murky.

Firstly, there's the problem in enabling taint mode to begin with.  Perl
*needs* to start with the -T switch in order to be effective.  Enabling
taint at runtime (eg, using Taint::Runtime) breaks many of taint's
assumptions.  If taint is enabled at run-time, then any data already read,
all of %ENV, all of @ARGV, and anything else that should be tainted, isn't.
 Perl has already walked through PERL5LIB and PERL5OPT and loaded up
untrusted modules and executed code.

Easy, one may say, Modern::Perl can just ensure that Perl was started in
taint mode.  It's a nice idea, but unfortunately it will stop people from
using Modern::Perl.  Taint mode, as a rule, is a pain in the arse.  It's
*worth* being a pain in the arse for the benefits it provides when you want
them, but it typically means, depending upon the developer:

	* One spends a lot more time thinking about data and validation
	  and regexps, and what to do when we validate without regexps, and
	  what to do when we have regexps which are not used for
	  validation.

	OR:

	* One sidesteps the whole taint problem by using bulk untainting
	  mechanisms which defeat the whole point of taint, but result
	  in extra work.

Because of the all-or-nothing approach to Perl's taint mode, if one wishes
to use Modern::Perl inside a module, then one needs to use taint mode for
the program that uses it.  Because taint mode is all-or-nothing, there are
some existing CPAN modules that just break (or require large workarounds)
when taint mode is enabled; these can't be used in a Modern::Perl program
either.

The end result is that for a lot of developers, they'll start to avoid using
Modern::Perl, because the taint headaches are too large.  I think it's much
better for Modern::Perl to do the right thing for the overwhelming majority
of cases, and allow individual developers to type the extra two keystrokes
for taint mode when they deem it necessary.

All the very best,

	Paul