it would be nice to have a "code not reachable" message

There isn't such a check for post-exit(), but Perl::Critic definitely has a rule for pre-use strict;. I guess one could write a rule that ignored subroutines...

Hmm,

Perl::Critic::Policy::ControlStructures::ProhibitUnreachableCode - Don't write code after an unconditional die, exit, or next.

Perl::Critic::Policy::Modules::ProhibitExcessMainComplexity - Minimize complexity in code that is outside of subroutines.

Oof. Updated.

I ran a little test script but my .perlcriticrc had got borked somehow so perlcritic ran with -5 (why is that the default, anyway?), and the rule has a severity of 4. I didn't notice anything wrong because "Code before strictures enabled" has a severity of 5, so it did throw an error. Thanks for the correction. (When I first went to answer the question I thought I remembered there was a rule!)

#!/usr/bin/perl
use warnings;

my $foo = 'bar';
use strict;

exit(0);
my $baz = 'quux';
[download]

$ perlcritic 1140661.pl
Code before strictures are enabled at line 3, column 1.  See page 429 
+of PBP.  (Severity: 5)
[download]

$ perlcritic -4 1140661.pl
Code before strictures are enabled at line 3, column 1.  See page 429 
+of PBP.  (Severity: 5)
Unreachable code at line 9, column 1.  Consider removing it.  (Severit
+y: 4)
$
[download]

The way forward always starts with a minimal test.