attributes (Perl in a Nutshell, 2nd Edition)

8.3. attributes

sub foo : method ;
    use attributes ( );    # Optional, to get subroutine declarations
    my @attrlist = attributes::get(\&foo);

The built-in attributes are:

locked: Setting this attribute is meaningful only when the subroutine or method will be called by multiple threads. When set on a method subroutine (i.e., one marked with the method attribute), Perl ensures that any invocation of it implicitly locks its first argument before execution. When set on a non-method subroutine, Perl ensures that a lock is taken on the subroutine itself before execution. The semantics of the lock are exactly those of one explicitly taken with the "lock" operator immediately after the subroutine is entered.
method: Indicates that the referenced subroutine is a method. This has a meaning when taken together with the locked attribute. It also means that a subroutine so marked will not trigger the "Ambiguous call resolved as CORE::%s" warning.
lvalue: Indicates that the referenced subroutine is a valid lvalue and can be assigned to. The subroutine must return a modifiable value such as a scalar variable, as described in the perlsub manpage.

There are no built-in attributes for anything other than subroutines.