you may want to have a look at the following tests

https://metacpan.org/source/DCONWAY/PPR-0.000021/t/ppi_statement_sub.t

and

https://metacpan.org/source/DCONWAY/PPR-0.000021/t/subdecl.t

I wasn't able to find examples of attributes with arguments, so better see the corresponding perldoc in perlsub#Subroutine-Attributes

Examples of valid syntax (even though the attributes are unknown):

sub fnord (&\%) : switch(10,foo(7,3))  :  expensive;
sub plugh () : Ugly('\(") :Bad;
sub xyzzy : _5x5 { ... }

Examples of invalid syntax:

sub fnord : switch(10,foo(); # ()-string not balanced
sub snoid : Ugly('(');        # ()-string not balanced
sub xyzzy : 5x5;              # "5x5" not a valid identifier
sub plugh : Y2::north;        # "Y2::north" not a simple identifier
sub snurt : foo + bar;        # "+" not a colon or space
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