#!/usr/bin/perl

use strict;
use warnings;
use feature qw/say/;

use vars qw/$VERSION/;

$VERSION = 0.01;

UNITCHECK {
sub _running_under_docreader {
	!! $ENV{PERLDOC}
	}

sub _running_under_tester {
	!! $ENV{HARNESS_ACTIVE}
	}

sub _running_as_app {
	defined scalar caller
	}

my $method = do {
	   if( _running_under_docreader() ) { 'doc'  } # reading docs
	elsif( _running_under_tester()    ) { 'test' } # testing
	elsif( _running_as_app()          ) { 'run'  } # running the application
	else                                { undef  } # everything else
	};

__PACKAGE__->$method(@ARGV) if defined $method;
}

sub run
{
   say $VERSION;
}

$ ./foo.pl 
Use of uninitialized value $VERSION in say at ./foo.pl line 36.