It seems Exporter (which Readonly, and many, many, many, many other modules use internally) has a weird feature (in my opinion a misfeature) where it does its own module version number checking.

Normally when you write:

use Module 1.23;
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... perl will catch the version number, and not treat it as an argument to Module->import, instead passing it to Module->VERSION to act as a version number check.

However, it seems Exporter makes an attempt to catch number-like arguments that perl has missed, and do its own version checking with them. Notice the difference between the following two examples; the first one where the version check is done by perl, and the second where it's done by Exporter.pm

$ perl -e'use Carp 100 ()'
Carp version 100 required--this is only version 1.26 at -e line 1.
BEGIN failed--compilation aborted at -e line 1.

$ perl -e'use Carp () => 100'
Carp version 100 required--this is only version 1.26 at /home/tai/perl
+5/perlbrew/perls/perl-5.16.2/lib/5.16.2/Exporter/Heavy.pm line 120.
BEGIN failed--compilation aborted at -e line 1.
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In your original example, this line:

use Readonly my $unprintable => 0x15;
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... is run as:

use Readonly undef => 0x15;
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... because the variable is undefined. So Exporter's version number check kicks in, even though the 0x15 was clearly never intended as a module version number.

Yet another reason to avoid using Exporter and use something sane like Sub::Exporter.