foo-test.pl

#!/usr/bin/env perl
use strict;
use v5.10;
use Test::More tests => 2;

require './foo.pl';

is( fooize('test1'), 'foo test1');
is( fooize('test2'), 'foo test2');

foo.pl

#!/usr/bin/env perl
use strict;
use v5.10;

sub fooize {
    'foo ' . $_[0]
}

if (not caller) {
    say fooize 'ohai';
}

1;