require a script w/o side effects

foo.pl

#!perl

use strict;
use warnings;

return 1 if caller; # before parsing args, output, input etc

print foo(); # side effect! 

sub foo {
    return 42;
};

foo.t

#!perl

use strict;
use warnings;
use Test::More;

use lib ".";
require "foo.pl";

is foo(), 42, "Foo is fine";
done_testing;