From @edwinst

$ ./perl6

class A { has @​.a of int }
Null PMC access in get_string()

$ ./perl6 --version
This is perl6 version 2012.09.1-30-g8230aef built on parrot 4.4.0 revision RELEASE_4_4_0

Seems to be REPL-specific​:

$ ./perl6 -e 'class A { has @​.a of int }'
===SORRY!===
Cannot call 'trait_mod​:<of>'; none of these signatures match​:
:(Routine​:D $target, Mu​:U $type)
:(Mu​:U $target, Mu​:U $type)

at -e​:1

From @usev6

As a status update​: The REPL with rakudo.jvm gives a NullPointerException​:

$ echo 'class A { has @​.a of int }' | perl6-j

class A { has @​.a of int }
java.lang.NullPointerException

Executing the command with -e the error message is unchanged (it's the same error when evaluating the code in the REPL with rakudo.moar)​:

$ perl6 -e 'class A { has @​.a of int }'
===SORRY!=== Error while compiling -e
Cannot call trait_mod​:<of>(Attribute, int); none of these signatures match​:
  (Mu​:U $target, Mu​:U $type)
  (Routine​:D $target, Mu​:U $type)
  (Variable​:D $v, Any |c)
  (Variable​:D $v, Mu​:U $of)
at -e​:1
------>

With parrot (latest version from rakudo star 2015.02 it's still a Null PMC access with the REPL​:

$ echo 'class A { has @​.a of int }' | perl6-p

class A { has @​.a of int }
Null PMC access in get_string()

I'll change the subject to mention the NPE on JVM.

@usev6 - Status changed from 'new' to 'open'

From @usev6

This works now​:

$ perl6-m -e 'class A { has @​.a of int }; my $foo = A.new; $foo.a = 1,2,3; say $foo.perl'
A.new(a => array[int].new(1, 2, 3))

Also, there is no NPE with the REPL​:

$ echo 'class A { has @​.a of int }' | perl6-j

I added a test to S12-class/attributes.t with commit Raku/roast@311b07e6b8

I'm closing this ticket as 'resolved'.

@usev6 - Status changed from 'open' to 'resolved'