Attributes are like post-it notes you can tack onto anything in Perl. The technical term is "out of band data" I believe.

Attributes are used extensively in Catalyst to declare the URLs to which a subroutine is bound, and I've used them to declare access control lists together with the subroutine. They work by giving you a callback when declaring the subroutine, so they are more or less syntactic sugar for:

Ultracool->declare_subroutine('frobnicate',sub {
  ...
}, qw(some more arguments));
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