Let's reduce that to the simpler:

subtype DateTime => as 'Object';

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... because that's all that's necessary to see what's going on. The problem is that Perl parses this in two different ways depending on whether DateTime has been loaded. (Or more formally, depending on whether there exists a stash called DateTime::.) If DateTime is not loaded, it gets parsed effectively as:

subtype('DateTime', as('Object'));

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But if DateTime is loaded, it can (at least potentially - these things are tricky to predict) be parsed as an indirect method call, i.e. like this:

DateTime::->subtype( as('Object') );

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This is in fact one of the reasons why it recommends picking a name like "MyDateTime" for your datetime datatype. Differentiating between the Moose type constraint name and the Perl package name.

FWIW, MooseX::Types gives you a much more sugary syntax for types.

perl -E'sub Monkey::do{say$_,for@_,do{($monkey=[caller(0)]->[3])=~s{::}{ }and$monkey}}"Monkey say"->Monkey::do'

In reply to Re: Moose::Util::TypeConstraints - Querying subtype() Caveat by tobyink
in thread Moose::Util::TypeConstraints - Querying subtype() Caveat by kcott

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