And there's a Perl 6 way of doing it:

Actually, what I had the back of my mind was Perl 6's subtypes. In my original code, each CheckFactory corresponds to a named subtype, and each Checkee corresponds to an instance of the subtype.

Zaxo's Tie::Constrained module, to which another monk helpfully pointed me, works more like an anonymous subtype, in which you have to restate the constraint every time you want to constrain a new variable. That may mean Zaxo had a different application in mind (where each variable had different constraints, and the need to define a factory function would have just made the module harder to use); or it may mean he bumped up against the same problem I did, and couldn't find a way to write a good factory function to churn out identically-constrained variables.

Cheers,

Markus