I want to add an optional method to a set of similar classes.

You could just import the method into the classes that need it.

For example, if you created a module like the following,

use strict;
use warnings;

package SpecialMethod;

BEGIN {
   our @EXPORT = qw( special_method );
   require Exporter;
   *import = \&Exporter::import;
}

use ...
use ...

sub special_method {
   ...
}

1;
[download]

then just say use SpecialMethod; from the various classes into which you want to import the method.

Update: Oh wait. That doesn't address your main question at all. I don't think I can give a good answer without knowing more about what you are trying to accomplish, but it sounds like a perfectly reasonable use of can.

Update: Added missing *import = line.