Easy. Imagine you have to sort objects. Objects written by someone who understands encapsulation (hence, unlikely to be a native Perl programmer). So, there's no way to compare two objects using an operator - you've got to call a method in one of the objects, passing the other as argument, for instance:

@sorted = sort {$a->cmp($b)} @unsorted;
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I doubt you can either use a ST or a GRT (I am one of the people who don't think GRT is a special case of ST) to sort this.