Why do some functions in Perl have to be called with parens and others don't?

An example to illustrate is the Synopsis of my own Test::Version.

use Test::More;
use Test::Version 0.04;

# test blib or lib by default
version_all_ok();

done_testing;

I don't have to include parenthesis on done_testing(); I can simply call it. However when I've tried to call version_all_ok; ( note: First attempt at Dist::Zilla::Plugin::Test::Version failed this way) I get an error. Why is this?

Update Perhaps my example is not quite as good as I've thought. The actual error I've gotten is

Bareword "version_all_ok" not allowed while "strict subs" in use at t/release-test-version.t line 19.

and here's the full code

#!/usr/bin/perl

BEGIN {
  unless ($ENV{RELEASE_TESTING}) {
    require Test::More;
    Test::More::plan(skip_all => 'these tests are for release candidate testing');
  }
}

use 5.006;
use strict;
use warnings;
use Test::More;

eval "use Test::Version";
plan skip_all => "Test::Version required for testing versions"
    if $@;

version_all_ok; # of course line 19, and version_all_ok() works here.
done_testing;

The following should be relevant snippets pulled from Test::Version 1.0.0 for exportation.

use parent 'Exporter';
our @EXPORT = qw( version_all_ok ); ## no critic (Modules::ProhibitAutomaticExportation)
our @EXPORT_OK = qw( version_ok );