How about marking the validation routine rather than the parameters, so you would have:

constraints => {
     image_name => [
     {
         constraint_method => 'file_format',
     },
     {
         constraint_method => 'file_max_bytes',
         params => [\100],
     },
     {
         constraint_method => 'image_max_dimensions',
         params => [\200,\200],
     },
 
     ],
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you could either add a method to D::FV that allows you to find out the name of the field whose contraints are currently being checked (which would involve rewriting the validation subroutines), or make it a convention that the first two args are always the D::FV object and the name of the field being checked.

So:

     foo => { constraint_method => 'bar', params => \@args },
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would translate to either:

     foo => { constraint => 'bar', params => [$dfv, @args]] },
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or

     foo => { constraint => 'bar', params => [$dfv, 'foo', @args] },
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(where $dfv is the D::FV object in question)

I'd prefer the former since you would then have a direct mapping to the Perl method calling convention:

bar($dfv, @args);
$dfv->bar(@args);
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