From @Siddhant

  $ cat error.pl
  class foo {
  has $.x;
  }

  my $obj = foo.new;

  $obj.x = 42;

  $ perl6 error.pl
  Cannot assign to a readonly variable or a value // needs a
line no. here
  in block <anon> at error.pl​:1

  $ perl6 --version
  This is perl6 version 2012.06-14-gdba6e9c built on parrot 4.5.0
revision RELEASE_4_5_0

From @moritz

Thanks for your report. It turned out that the problem was generally
with assignment to non-variables, not just to read-only accessors.

Fixed in rakudo/rakudo@1ffd6cf137, and tested
in integration/error-reporting.t.

@moritz - Status changed from 'new' to 'resolved'